How to stop an evil revanant

[NecroDancer]

long story short, last year in our campaign (IRL) our party overthrew an evil baron (lots of horrible side-effects). My character took out the baron's adopted son/murderous, creepy, stalker, bodyguard (he was evil and totally deserved it the DM confirmed this) and now the son is back as a revanant.

We encountered him in today's session and he is dead set on killing me (chaotic stupid is a horrible alignment, even if you are tempted don't do it, you will regret it, I certainly did). After a tense (BUT EPIC) mid-air battle we knocked him out for info but he killed himself and escaped (he has a demon arm that regrows instantly and shoots fire).

What should I do to stop him from killing me and the party? my character will do anything (he's NE from a curse) to stop him but seeing as he can always regrow his fiery demon arm he can just kill himself temporarily.

my current plan is to find or pay for 300 gp diamonds and get the party to kill me (putting the revanant's soul to rest) and then bring me back to life (i don't want to waste any diamonds we already have).

[JackPhoenix]

How did he escaped after killing himself? You've destroyed the corpse or left it lying around? Does the arm work when he's "dead"? If not, kill him, restrain the body (staking it in a bog for a historic inspiration, or just putting on chains and entombing it). When he revives, he's still entombed, even if he kills himself again, he'll still be entombed when he revives again. Wait for a year. Problem solved.

If the hand throws fire even after he's "dead" and thus can destroy the corpse, find a way to stop that... put it in a bucket of water (or even better, holy water) until you prepare the grave, do the thing above. Bog may be a better option in this case, it's wet so the fire won't do anything, but it's not like dirt or stone burn either.
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[NecroDancer]

so the arm regrows super fast, my party cut off the arm and then attached some metal to the stump before the arm could grow back. However the revanant just regrew his arm until he was able to burst out of the metal or die from the injuries he suffered from the process.

The DM has a thing where our enemies get stronger as the party get's stronger, for example we killed a cult leader and than left, we gained a few levels and know we heard that the cult leader was raised by her minions and apparently is doing stronger magic. basicaly the longer we ignore the revanant the stronger he becomes (so we can't just insta-kill him at level 10).

We have contemplated putting his arm into a bucket of water but we are unsure on how to keep it there.

[JackPhoenix]

Knock him out and keep hitting him with fire or radiant damage so he won't regenerate, but not enough to destroy the body. Or someone may hold him, or tie him up. If the DM uses MM revenant, he's got one year to take his revenge, so the burial doesn't have to last forever... but it sounds like the DM may find a way to free him before that.

Metal "sleeve" to fit over the arm may also work... shouldn't be that hard to get someone to metal a metal tube with a cap on one end.

It shouldn't matter if he dies, as long as the body isn't destroyed, his spirit won't be free to possess a different body.