Sil'as Dahcsnu is a monk of the Dagon'hai order. He can be found in the Tunnel of Chaos. He has no purpose during the quest, but his talk options end up with him questioning your magical powers, and he will then cast a spell on you that. When your Magic level is below 80, this will cause your character to dance and shout, "Argh! I can't stop dancing/headbanging/jumping!" However, if you have level 80 Magic or higher it will backfire onto him, making him dance instead.
The spell cast by Sil'as Dahcsnu backfiring