There are 3 demons. Two of them are the same (even have identical names IIRC), and the third is different. The third even can't hold you (?) (he has some other nasty abilities, dont worry
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My way to deal with them was to enrage (immunity to hold [img]smile.gif[/img] ) or send in some Mordenkainen's swords lead by a wizard's eye [img]smile.gif[/img] The result was amazing in both cases - lots of gibs and a demonheart lying lonely on the ground.