I would approach it one of two ways.
Option 1: Give them 0 XP the first time, but full monster xp the 'last/final' time.
Reasoning: You could argue that the spirits aren't being defeated, in which case the PCs would get 0 xp until they are able to utterly destroy the undead (in such case, I would make them worth "normal" monster XP; call them regular monsters but with only 1 hit point, and the power "Daily Revivification: At midnight the spirit warriors regenerate back to max hit points. If the spirit warrior received radiant damage, this power no longer functions.")
Option 2: Give them xp the first time, and minor quest reward xp the last/final time.
Reasoning: you could argue that they aren't defeated, but the PCs did overcome the encounter threat. In which case, I would say to give XP the first time, but not subsequent times (because subsequent times, the PCs KNOW that the undead spirits are less of a threat compared to their skill... i.e. minions. And then give a minor quest reward for the permanent radiant destruction of the spirits. Disclaimer: I would not use this justification if they aren't minions, since not being minions makes them more of a variable threat each time).
Basically, my point is, just remember, the moment they stop being "reasonable threats" to the PCs (be it by virture of tactics/skill/forehand knowledge) then they turn in to a 'bag of rats' which are not worth any xp. So you would give them xp either the first time they kill them, or the last time they kill them with radiant damage, but only one or the other -- that's how i'd do it anyway.
Personally, I lean towards option 2. But I'd also take in to consideration the chance of the PCs even encountering the situation a second time. if I have strong reason to believe that the PCs would never go back there again (i.e. they'll be far removed/busy with other things/etc) then I'd instead go with option 1.
Of course, I don't give out XP after each encounter, I give it out per session, so I could fiddle with it depending on how they ended up acting.
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Option 1: Give them 0 XP the first time, but full monster xp the 'last/final' time.
Reasoning: You could argue that the spirits aren't being defeated, in which case the PCs would get 0 xp until they are able to utterly destroy the undead (in such case, I would make them worth "normal" monster XP; call them regular monsters but with only 1 hit point, and the power "Daily Revivification: At midnight the spirit warriors regenerate back to max hit points. If the spirit warrior received radiant damage, this power no longer functions.")
Option 2: Give them xp the first time, and minor quest reward xp the last/final time.
Reasoning: you could argue that they aren't defeated, but the PCs did overcome the encounter threat. In which case, I would say to give XP the first time, but not subsequent times (because subsequent times, the PCs KNOW that the undead spirits are less of a threat compared to their skill... i.e. minions. And then give a minor quest reward for the permanent radiant destruction of the spirits. Disclaimer: I would not use this justification if they aren't minions, since not being minions makes them more of a variable threat each time).
Basically, my point is, just remember, the moment they stop being "reasonable threats" to the PCs (be it by virture of tactics/skill/forehand knowledge) then they turn in to a 'bag of rats' which are not worth any xp. So you would give them xp either the first time they kill them, or the last time they kill them with radiant damage, but only one or the other -- that's how i'd do it anyway.
Personally, I lean towards option 2. But I'd also take in to consideration the chance of the PCs even encountering the situation a second time. if I have strong reason to believe that the PCs would never go back there again (i.e. they'll be far removed/busy with other things/etc) then I'd instead go with option 1.
Of course, I don't give out XP after each encounter, I give it out per session, so I could fiddle with it depending on how they ended up acting.
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