A real pain in the ass to defeat in Baldur's Gate. Most players are willing to take the risk and the reputation loss to gain his three incredible weapons, but he's basically impervious to magical attacks and moves like the wind. The preferred method is generally to zap him with a wand of monster summoning as many times as you possibly can to keep him in one place, and then murder him at ease with archers. It's still difficult, but worth it.