Up to you. Mind that getting xp should not actually from a kill, but from challenges. That's why a level 20 character shouldn't get xp from killing a thousand goblins. But he might get xp if he's doing it as a general that command a village of commoner to defend themselves from the goblins though! Convincing a king to do peace treaty with the hostile neighbour should give the party exp, etc. So if it's me, surviving the ring for the first time would give them some xp, but beating it multiple time shouldn't, because the xp is from the challenge of figuring out how to beat the ring. and destroying it should give more xp, from the challenge of figuring how to permanently destroy it.