Lassal is in the room of spikes after you beat tanova. Paladins and mace of disruption +2 helps. Also make sure you equip the amulet of power to your mage. This grants protection from negative energy (level drain) as well as vocalize which means you cant be silenced. The spell of protection from negetive plane energy helps. Try breaching tanovas defenses and keldorns true sight kills her double.
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ShadowMaster of the Theives of Darkness.
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