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It gives the user +2 wisdom
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Oinchack'Olp plane shifts out right after you return it, and there is no dialogue option I know of to talk him out of it. If you want it, you have to either never speak with him again or fight him but either way that sacrifices the XP.
**spoiler**
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You do have "1" round before Oinchack'Olp plane shifts out after you give him the stone. You have to maximize damage in that one round. Since he does not speak to you as you approach so you can set this up rather easily. This worked like a charm for me, I summoned two undead (I think they were elite boneguards) and used them in conjunction with most of my party to surround Oinchack'Olp. I put my 2 best missle weapon members within range but apart from his thralls. I paused the game and then ordered the entire party to attack. Before unpausing I then clicked on the party member that was the least damage inflicting and ordered them to speak to Oinchack'Olp. I then unpaused the game. He gets the stone, I get the XP, and then he gets cut down "like a blade through grass." The thralls still attack but they are fairly easy to take down. When you loot the body, there's the stone. You also get the XP for killing him as well.