First, let's meet the revenant. Not only does it always exist, the revenant is also undead. Undead are extremely difficult to permanently disable. This gives adventurers a list of things they can't do. They can't permanently command it to do nothing, paralyze it, hopelessly poison it, or permanently drain its abilities to the point it can't act. Worse, they can't subdue it by reducing its hit points to 0 since it is immediately destroyed; they also can't subdue it with non-lethal damage. All of the following undead traits make the revenant very difficult to destroy.
- Immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms).
- Immunity to death effects, disease, paralysis, poison, sleep effects, and stunning.
- Not subject to nonlethal damage, ability drain, or energy drain. Immune to damage to its physical ability scores (Constitution, Dexterity, and Strength), as well as to exhaustion and fatigue effects.
- Immunity to any effect that requires a Fortitude save (unless the effect also works on objects or is harmless).
- Not at risk of death from massive damage, but is immediately destroyed when reduced to 0 hit points.
- Not affected by raise dead and reincarnate spells or abilities. Resurrection and true resurrection can affect undead creatures. These spells turn undead creatures back into the living creatures they were before becoming undead.
These traits suggest one out, casting resurrection (cleric 7, 10,000 gp material component) on the revenant. This is a bad choice even for high-level characters because
A soul can't be returned to life if it doesn't wish to be. A soul knows the name, alignment, and patron deity (if any) of the character attempting to revive it and may refuse to return on that basis.
The murderer's victim may not be cooperative in coming back to life just so their murderer can escape the torment of a revenant. A more reliable high-level solution is to imprison the revenant on a demiplane created specifically to be its prison. An appropriate demiplane can be constructed with a permanent create demiplane, lesser (cleric 7, sorcerer/wizard 7, 18,000 gp material components)
Since a revenant is CR 6 I'm going to assume a party of adventurers seeking to disable it are somewhere around the same level. If acquired on scrolls the necessary spells cost 21,400 gp. If destroying the revenant is particularly narratively important to the story, the adventurers may be willing to spend a significant amount of their resources on its destruction. A party of four 6th level characters should have a combined wealth of about 64,000 gp. Creating a permanent demiplane to hold this revenant would cost just over a third of the party's resources.
The party will need a sorcerer or wizard with 17 intelligence or charisma (or at least 17 for the two hours it takes to read create demiplane). Activating the scroll requires a DC 14 caster level check, which the reader should fail no more than 35% of the time. If they do fail there's a small chance (DC 5 Wisdom check, natural 1 always fails) that a mishap will happen; see the rules for scrolls. Otherwise they can attempt the same thing all over again and spend another 2 hours reading the scroll. The writing disappears from the scroll when the scroll is successfully activated.
The party will need to subdue the revenant (physically overpower) while the spell is being cast or create the demiplane first and then use some form of extradimensional travel (more expensive). The party will need to hold hands (and hold the revenant's hands) at the end of the spell to travel to the new demiplane. The caster can then eject everyone except the revenant one at a time as a standard action. The caster will end up exposed to one round alone with a revenant on another plane of existence.
The revenant can also be defeated by an ordinary mirror, if we read its entire description.
- Self-Loathing (Ex) When confronted with its reflection or any object that was important to it in life, a revenant must make a DC 20 Will save to avoid becoming overwhelmed with self-pity. This condition renders the revenant helpless, and lasts until the revenant is attacked or sees its murderer. If a revenant resists becoming overwhelmed, the revenant becomes obsessed with the source that triggered the saving throw and does everything it can to destroy it, reacting to the trigger as if the trigger were its murderer and gaining bonuses from its reason to hate ability.
Bury it deep someplace it won't be found or disturbed, preferably under at least a few feet of soil or stone. But what's the fun in that?