I was pondering this one just the other night. Luckily, I don't hand out XP until the next session, so I have a few minutes to ponder.
The PCs fought two abishai (draconic, devilish servants of Tiamat), downing one and driving off the other. They already saw one go down with a critical hit (arrow in the eye), and then get back up two rounds later to fly away, so they know that these things regenerate, and they saw that their non-magical weapons weren't cutting the mustard, so they know that they have damage reduction. I left the encounter at a "what the hell were those things?" cliffhanger, and there's a couple of things they
don't know:
a) Unless it occurs to them to use holy water or a
blessed weapon, they have no way of killing the one that's still there (these mothers have 5/+1 damage reduction, regenerate 4 hp a round, and only holy water / blessed weapon damage is permanent).
b) The abishai have several nasty spell-like abilities, none of which require somatic gestures to use, so if (as I suspect will happen) they bind this thing and try to question it, it's going to try to
charm somebody into releasing it, and - failing that - summon baatezu (dretches, I believe) to make their lives miserable.
This presents several intersting XP-related questions. First, the dretches: XP for them? This is an easy one, the DMG is real clear that summoned monsters are woth no XP, as the ability to summon is taken into account in figuring the Challenge Rating of the creature that's doing the summoning. (Which raises an interesting question - should I even award
any XP for the abishai, until the PCs discover and defeat the villain who's summoning
them?
Hehehe...)
But, here's the sticky thing: What about regeneration? Sure, the PCs defeated the beasties, from a certain point of view - they reduced
both of their HPs to negative numbers, at one point or another - but they're not dead, they're still a threat, and I have a sneaking suspicion that the one they've got is going to get away inside of five minutes, next session. You could make the case that they've bested the abishais in combat, and deserve XP for this (which interpretation I am leaning towards), only there's that nagging voice that says
it's in the Challenge Rating... After all, an ability like regeneration surely figures into a CR calculation, and the thing that makes that such a pain isn't that they're healing damage during a battle, but that they can get back up afterwards unless you do the
right kind of damage.
The answer is really simpler than all that, I'm just playing devil's advocate (literally), here. Here's how I handle it: The PCs get XP for the battle -
once - whether they have to "re-kill" the things eight times, or they figure out the "magic number" and kill them "for real", or they get away and are gone. What they
won't get XP for is downing them more than once in a battle, or for driving them off only to have them fly overhead out of bow range to heal for a minute and come back for more. It's all gotta come down to DM common sense, in the end, and what "feels right" for your game.