Depends on how strongly you want that fate to be. Perhaps by twist of words, those rocks were actually caused by finn's chain of action somehow, thus technically they died by finn's hand. Or jake did survive that fall, only to die cracking their head on a banana peel finn threw randomly one day

You choose ofc, and its a genius move for writing. You can use it to make a character obsessed with controlling their death so they can never die (kinda like Odin in GoWR) or they can be some random hermit who’s accepted their fate and are just living in peace until that destined day.

You could also however make it a curse where they cannot die but they can get dangerously close to dying regardless of what it is until ofc their enemy comes to kill them. Or better yet, they’re proper invincibl- I mean [title card] but if their enemy is killed bc they obviously don’t share this destiny, the other person either can never die or will HAVE to die.

If Fate is an absolute and cannot be defied, then yes, having a legitimate prophecy of a specific future makes you immortal until that prophecy comes to pass.

However, prophecies tend to rely heavily on metaphors, abstractions, and double meanings, so a prophecy saying "Jake will die at Finn's hand" could mean that Finn will kill Jake, Finn will help Jake to fake his death, Finn botches Jake's brain surgery (making him brain-dead), or it could mean that some rando chops off Finn's hands and makes gloves out of them and strangles Jake to death.

Prophecy is a nasty, finicky thing that will screw you over in every way you can imagine, and should not be relied upon for protection.

depends how strong mystic forces are in your world, Jake in a low mystic world could die prematurely or survive the battle if he never met Finn, but generally the world nudges the events into place.

Or you could go hard mystic, where Jake falls 100ft and is crippled having a broken back, and is generally rendered braindead by taking a spike through the frontal lobe. But the world heavily pushes Jake towards his destiny, maybe a commander saw him and felt bad, so brought him in his company to care for him, which takes him straight into battle where Finn finds and kills him, smh like that.

In Greek drama, it’s usually the act of trying to avoid a prophesied fate that causes it to come about - see Oedipus.

There’s basically three sorts of prophesies:

Destinies will happen, no matter how much the person tries to avoid it. (Jake jumps off the 100’ cliff into a pit of stakes. He’s badly wounded, but survives. Finn hears Jake’s cries of pain, helps him out of the pit, then delivers a coup de grace to end Jake’s suffering.)

Warnings* will happen unless the person heeds them. (“Beware the Ides of March” says the soothsayer. Caesar heeds the warning, avoids the Senate, and is not assassinated.)

Finally, there are instructions: a prophecy that tells you how to do something, not that it will happen.

My favourite version of this is from The Dark Crystal:

When single shines the triple sun,
What was sundered and undone.
Shall be whole, the two made one
By Gelfling hand, or else by none.

It doesn’t say the Crystal will be made hole, it just explains that only a Gelfling can do it.

Have you been watching Rick and Morty?

A factor is also how simple are the profechies to understand? Are we talking Shakespeare/Tolkien, where the meaning is a revealed to be different then originaly thought?

Maybe Jake just falls off thd cliff and into the spikes but still survives but is badly wounded and that makes Finns job a lot easier.

Yes, but remember, immortal doesn’t mean invincible

So you can still be hurt, badly but you just won’t die

Idk if this is really a world building question.

If a prophecy, which we assume is true and will happen, is that Jake dies by Finn, then Jake wouldn’t ever fall off a cliff to begin with. If he did fall off a cliff it would never be a cliff high enough to kill Jake.

It’s not that Jake can’t die, it’s that he won’t die, because he’s supposed to die by Finn.

When you make prophesies you’re creating a deterministic, or probabilistic universe, as well as free will or no free will.

Deterministic is if the prophecy is true, someone saw the future, and the future can’t be changed no matter how hard you try, you have no free will. You can change the free will portion a bit by saying he’ll die by Finn but not exactly how, but it’s not truly free will in that case.

Probabilistic prophecy is the current future is seen, but can be changed by your actions. Therefore the character has agency, they have free will to make their own choices and choose their own path, and if they don’t change, the prophecy will come true.

So prophecies should be used as a means to motivating action from the character, in that case.

Prochecies aren't usually set in stone. At least in my world.

Depends on how tight your Fate is. There is a variation of the tale of the king who killed his son and evaded the prophecy that his son killed him. But eventually die at his hands. The variation says: the king raised the child in a loving manner, hoping to not die by him. His son as an adult, knows the prophecy and leaves. Then the king sees his son leaving and dies of a broken heart.

I would say they are immortal, yeah.

How I would handle it:

I'd probably treat it as "Unless it's their fated time to die, they cannot die, but by the least offensive method to reality"

So if a sword's swung at him, the guy swinging it just misses. If a gun's fired at him, it misfires. If he falls off a cliff, it just-so happens there's a ledge he, by some improbable twist, lands on.

By the same token, they can't do anything to avoid their fate. Mentally, they "know" this (just like you or I know gravity is real) and don't even really try very hard. If they (or someone else) does, fate "fixes" it so things carry out as they should.

How much people know about this state of affairs is purely down to the setting and individual situation. It could range anywhere from totally unknown and inexplicable to "LOL watch Johan the Fated chug a gallon of Hemlock and then dance a turn!"

Possibly fate could punish egregious violations by the fated person or those near them.

So for me what this would mean, is that Jake Will be in the final battle, and he will fight finn and he will die there.

He is not immortal, but fate will conspire to ensure the prophecy becomes true. Thus if falling 100 ft off a cliff into a pit of spikes would cause him to die (as it reasonably would in most cases), fate prevents him from doing that. Fate only cares about preserving him enough that he can meet his destiny which means that fate will typically make the smallest possible change required.

Fate also doesnt like being messed with and attempts to exploit the nature of your prophetic fate typically get you dunked on.

In the Icelandic Saga of Arrow-Odd there was a prophecy that Odd would be killed by a specific horse. So he killed the horse and traveled the world for three hundred years before deciding to return home and visit the horse’s grave.

Where he got bitten by a viper nesting in its skull.

The classic Greek drama way of handling this would be that this character's effort to avoid the fate would actually make it come true through dramatic irony.

Also consider Macbeth, where he thinks he's Immortal because of the prophecy, but it turns out someone technically fulfills the prophecy because of a detail he didn't know.

Prophecies being confusing is also pretty typical. Is it possible it doesn't mean it doesn't mean exactly what it appears to mean? He could avoid this guy Finn, but maybe dies as a result of something Finn did, even if he isn't present. Maybe he ends up getting killed by a different guy whose middle is Finn. Maybe he could accidentally get sliced by some stabilizing fins on some kind of boat?

I think the key is to use a prophecy well, to try to fulfill it in some unlooked-for way.

I would say that character actively fighting his fate to end things before fate is fulfilled could do so. Like he could just slit his throat now and that's it. You should be able to dodge fate like that. Everything that is outside the characters control should naturally push person towards that fate. The more unreasonable he acts to circumvent fate, the more hurt he should get, but steer away from it. But if there is some reasonable circumstance how he can still finish up where he was meant to he should.

If the character starts running to bullet storms, he should get hit quite a bit, maybe survive, and get better, maybe die if it is too unreasonable. Use your best judgement.

I'm reminded of that comic of the princess fated to die younge...so takes up extreme sports or something

Jake wouldn’t fall into the pit of spikes in the first place (unless Finn pushed him in). If Fate wants Jake to die by the hand of Finn she’ll do whatever she can to keep him alive until then.

Sorta.

There's a show I remember where the main protagonist how he would die in the eyes of a witch.

As a result, he basically got plot armour as regardless of any dangerous encounter he faced, he would at always come out alive.

Of course, this doesn't mean they're invincible.

In fact, this is the cheapest form of immortality in my personal opinion.

Like you can still get cancer, which can either last with you up till the point you die or if you fight it, you chances of beating it are certain.

But you can also get a wide variety of different ailments and injuries.

Meaning you could be just a living brain in a jar upto the point of your death.

It honestly depends on how clear the prophecy is on your state of condition to point of your death.

If you're the type of person who throws themselves off a cliff or onto spikes because a prophecy says you'll only die to the hand of one specific other person, then such a prophecy won't be made about you.

You pretty much can't outmaneuver or trick fate. A prophecy is less "this now has to happen no matter what" and more "this is what will inevitably happen".

There is no such thing as an absolute prophecy, fate is merely an additional force weighing in on the world.

If Jake was bound by a fate of average strength to be killed by Finn, he'd only be slightly more likely to survive other harm. 

Nonetheless, a fate powerful enough could theoretically make him impervious to any other cause of death. However, each time a thread of fate is tested, it is weakened, and such a fate would grow weaker every time he grazes death

I'd read a book like that where something caused that battle to never happen to Jake is just forever immortal.

You have to change your concept of time from the many worlds theory that has flooded the entertainment sphere.

Look at a movie like 12 Monkeys. It is a perfectly circular time loop which cannot be altered.

If you are destined to die in a battle with a specific opponent, it's because that is what has always happened and will always happen. You never fell off a cliff. You would never hit by a car. You were never poisoned. You were never mold to death by a wild animal.

You survived through your life through a series of events that led to the final battle. Where you died.

It doesn't matter how many times you try to alter the loop. It is unalterable. Your future is fixed just as much as your past.

I usually try to avoid legitimate prophecies, unless the sage uses some kind of science to correctly guess some cause and effect. ("If you set sail now, you will die" "Because the poseidon cursed me?" "because this cloud formation usually precedes a big fucking storm")

If we introduce such a powerful concept as "Fate" into our world, we could also introduce necromancy or reviving magic, thus allowing for Jake to die more than one time. The second time being at the Hands of Finn.

Your Prophecy "Jake will be killed in the final battle by the hand of Finn" sounds an awful lot like it pertains to Ragnarok or Armageddon. So both of them die and much, much later get revived during the endtimes to face each other in the "final battle"

Also we got the Harry Potter method of confusing who is actually meant by the prophecy. It could be an entirely different Finn who kills Jake. For example the one who set up the pit of spikes in the first place.

Others also mentioned the problem of defining "Death" and "being killed". Maybe "being killed" means your bloodline ends, or the last memory of you gets erased. Or Finn wrote the prophecy themself, which causes Jake to believe themself to be immortal and do the 100 ft jump out of overconfidence, killing Jake.

"Perhaps the prophecy didnt include that you'd be revived right before that battle"

The character could very well die but come back to life later on

There are no prophecies. There is what will happen, and what will not happen. Death knows all, he has lived and witnessed the passage of every life and knows how it begins, and ends. He also exists in the places where what wasnt is. Alternate universes and all that. He knows that in a competition against the legendary mercenary Rolinjor, he will loose a dual, but the him that fights Rolinjor doesn't.

That is my world however. Destiny is unavoidable and any attempts to avoid it will only solidify it, and accepting it also solidifies it. If an Oracle were to tell someone that they will die of old age, and that person went and killed themselves young, the Oracle was lying.

Bug given you're premis, IF prophecy in your world works verifiably 100%, then all attempts to divert prophecy will only cause it to eventually happen. Someone or something saves Jake from the spike trap, or resurects him to fight and die at the right time or place. But if you really wanted you could explore the idea that literally nothing beyond the prophecy could kill or harm him, and then you see him shrugging off mortal wounds like some sort of invincible warrior, only to fall to a simple blade, fated to ne the one to kill him, out of thousands of others that pierced his heart, only one completes that cycle.

Well the most interesting part about prophecies is often how people react to them. So in the example “x must kill y or vice versa”, the prophecy might actually be averted if one of the participants has delusions of invulnerability due to knowledge of the prophecy and so behaves recklessly (ie Jake falling into spikes). Otherwise we could go the Harry Potter route where one of the participants prevents any of their allies or underlings from killing their rival because they believe they must fulfil the prophecy, which in turn leave them vulnerable.

In general I would say that no special protection is granted by such a prophecy, but that at the very least there will be various factors working together to keep participants alive up until that fated point. None of that will help if they aren’t trying not to die though.

I personally avoid anything to do with future sight and time magic in general when writing. I feel like it typically breaks things. 

However: 

Direct, specific prophecy in a world where prophecy is certain and literal is boring. If you make it as such then any suspense or lead up to anything except that final battle is meaningless since the reader knows you have explicit plot armor. 

It's much more fun to have the reader guessing at how that prophecy is to be interpreted, or if it is reliable. I like the idea of oracles seeing possibilities maybe even more likely probabilities better than seeing certain futures. You could even make oracles powers be rooted in math if you took that route lol. 

I think you could look at it a few ways.

If fate is exclusive? Then Jake literally cannot die any other way so accidents and assassinations fail until Finn kills him in the final battle.

In another, the prophecy is just predictive. Meaning it only says what will happen if Jake reaches that battle but offers no protection. So he could still die from a fall or illness and the prophecy simply never comes true.

Another is Jake isn’t immortal but he’s “death-proof” until destiny’s moment. Lethal situations tend to narrowly fail, injuries happen but aren’t fatal, and events subtly push him toward the final battle anyway. The prophecy doesn’t make him safe but it makes his death inevitable, just you know… not yet. In this version, fate doesn’t protect him, it merely delays collecting what it’s owed.

So Macbeth basically lol

"None of woman born shall harm Macbeth." and he goes on thinking he's immortal

Prophecies are tricky bitches (Oedipus Rex is a great example) so it can all be twisted hither and thither for whatever literary purposes you're working toward. All things serve the beam the story.

If I remember well, Tryndamere in League of Legends is kind of a nordic warrior who has a prophecy that he'll die peacefully. He wants to die honorably in battle, so he's always fighting but never dies. In game, his ultimate ability makes him invincible for a certain time.

It all depends of how you want to work with it, there are plenty interesting outcomes.

destiny determines the entire path to get there. if his destiny is to die in the future, he simply wouldn’t be put into an unsurvivable situation in the first place. so, either luck somehow saves him in exactly how the fall injures him (but doesn’t bend the rules of reality in doing so), he Frodo’s the fall by catching himself on the edge of the cliff to be pulled (or pull himself) back up, or he simply doesn’t fall in the first place.

In Twissen, the prophecy is a lie - plain and simple. Prophecy, and future-sight in general, are impossible. The closest you can get is a form of seer that can maybe predict a few actions into the future by analysing the present magically - and they are immediately stymied if there's another seer opposing them.

There are, however, types of magical protection that resemble it - for instance, you could make someone magically immune to damage except from a specific category of sources. They're hard to create, and there are ALWAYS loopholes (if nothing else, the anchoring point of the spell can always be broken) but until the spell is broken it will continue to protect them.

Jumping off a cliff is still a bad idea though, because the enchantment rarely includes protection from yourself.

-

In Urbania the prophecy was true, especially if it comes from The Oracle, but there is a generalized exception - the Fatecutter is a sword that cuts the strands of fate whenever it is wielded. No prophecy can ever take its existence into account, and whatever they say will happen is subject to change the moment Fatecutter gets involved. So, again, don't jump off a cliff - if the prophecy is still true, you'll survive (although you might break all your limbs and wind up not being much use in the final battle), but the threads may have already been cut!

That entirely depends on how prophecy works in your world; do you want such things to be set, predetermined, and infallible, or is there a way to change fate? Does someone knowing about a prophecy allow them to subvert or change it? If not, why not; what's the point in knowing?

That's how it works for Hellboy, he literally has a healing factor until he defeats his destined nemesis

Could be that Jake can die normally, but if he lives long enough to battle Finn, he is guaranteed to die by his hand at the fated moment?

It depends on how powerful you want fate to be in your world, and if you interpret fate in the “pull the sword out of the stone and become king” or “only this blade will harm him” way.

Honestly, it could even be both, predetermined outcomes for specific events and designated conditions for things to happen. They don’t have to be mutually exclusive.

Prophesy is the will of a god, but if there are multiple gods or other comparable powers involved they might not get their way.

If you're talking a prediction, a vision of the future, then the person simply will not have that lethal fall. They and those around them will make choices that happen to lead them to their fate, and random events will too happen to work out so that the predicted moment occurs.

I think if you want someone who is invincible to all but a certain thing you're looking at enchantments like the Achilles or Baldur myths.

Free Will gets in the way of accurate prophecy.

Until it happens NOTHING is set in stone, so the best a Prophet can give you is multiple choice.

If the prophecy works in english, the perhaps "by" means "nearby" not "by means of". Finn does not kill Jake, but Finn's companion does. Maybe even the one called "the Right (or Left) Hand of Finn" . Maybe the Hand of Finn is the title of a painting, statue, tapestry, or oxbow lake.

I mean your world works how you want it to, but that's not really how destiny works imo.

Destiny/fate is about all events happening in the proper sequence for a specific event to take place. The character in question isn't immortal because they can still die like any other person. If they get shot in the head or burned alive or what have you, they'd still be able to die. But the universe sets things up in a way to where those things never happen. If they do get shot in the head, the bullet misses sections of the brain that keep you alive. If they do catch fire, something puts it out before vital organs are harmed. Immortality is the inability to die, if anything you're character would just be extremely lucky.

So I'm gonna objectively say no, they are not immortal.

perhaps, but they can still have all their limbs removed and delivered to the destined one

This really depends on what "fate" actually is as a force in your setting.

In Wheel of Time for example there is an intelligent force, the wheel/pattern itself, trying to bring about certain events. So prophecies are created in an attempt to modify people's behavior in certain ways AND if things get off track intervention can happen. So, yes, events will transpire to ensure that the characters end up where and when they are supposed to be, even if the characters themselves don't exactly enjoy the path getting there. So getting hit with a non-deadly arrow or having a building fall on you are perfectly valid events. Incidentally alternate timelines are a thing, so you can see how at least one set of events happened differently.

That said for things I work on I personally dislike the vague prophecy trope and at best you would get a "this is how things are on track to happen right now", but that could change.

A little disturbing that you used fratricide as your example.

But setting that aside, it depends on whether or not prophecies actually matter in the story, and how specific they are. 

For example, if prophecies are bullshit, then Jake might be killed in a duel with Marceline due to the prophecy giving him false confidence. If the prophecy isn’t strict, Jake could be killed by a anyone named Finn, or even just be impaled by the fin on the metal statue of a shark. 

My setting doesn’t actually have prophecies that end up being fulfilled. The only prophecies are ones that are in an unspecified future, which exist as part of belief systems.