SPOILER
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Actually, you need a +4 or greater weapon (Crom Faeyr, Carsomyr, The Wave, several weapons in the underdark, or the flaming sword from the druid grove (+4 v.s. undead)) and a protection from undead scroll (or protection from magic if you are more paranoid). Give both to your best fighter, have the rest of your party out of sight (by the door), open the tomb, and read the scroll. The protection from undead scroll makes him ignore you; the protection from magic scroll make you immune to all his spells/special abilities. Just whack him until he drops (both times).
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Death is only the beginning...