As a GM, I use some revenants for my campaign. Those revenants all have class abilities and 2 are spell casters (an abjuration mage and alchemist artificer). The "problem" (it's not a problem, but an "and what if they do that?") is that they can use the Protection from Energy spell, so they can have fire resistance.
They have a Regeneration feature, with the following rules:
Regeneration. The revenant regains 10 hit points at the start of its turn. If the revenant takes fire or radiant damage, this trait doesn’t function at the start of the revenant’s next turn. The revenant’s body is destroyed only if it starts its turn with 0 hit points and doesn’t regenerate.
My question is: What happens if they gain fire resistance (or radiant resistance)?
Would they lose the weakness that prevents Regeneration from working when fire damage is dealt?
Best Answer
By the rules, resistance doesn't matter
Revenant's regeneration is suppressed when it receives fire or radiant damage. Since the revenant still receives fire damage, the resistance doesn't protect it from losing its regeneration, except in the unlikely scenario where the revenant would take a single hit point of fire damage before the resistance was applied (in which case the revenant'd take no damage and therefore wouldn't lose their regeneration).
Being the GM of your game, it's up to you to figure out how you want these revenants to play out. As a general caution, many rules that applied to a monster may stop making sense after the monster is augmented with class abilities and other such new features. You've already changed the creature so I wouldn't worry about being careful to respect the original regeneration rules.
However, if you change the revenant so that it can gain fire resistance, and choose to rule that fire resistance also protects the revenant from losing its regeneration, you ought to give your players a fair idea about this and make sure they have the means to combat the revenant. They are near-indestructible without fire or radiant damage, and depending on the party's composition, the player characters might have inadequate means to fight if one of these is ruled out.