Ertuu is a dark lord of the Abyss. He towers over most demons being 12ft. Most balors use a flame sword and whip, but Ertuu uses a magical electric sword and whip. He is immune to fires and electricity, and used to being in charge of vast demon armies.
Ertuu was around when the wizards first created the Crystal Shard and he had hoped to gain it for himself, but the wizards lust for power was too great. In finishing the creation of te crystal shard all the wizards present were killed and thier life force (magic) drained into the shard. Ertuu was destroyed, but since he was on the martial plane he was only banished back to the Abyss.
Ertuu waited years for the Shard to re-surface, and once it did he set his plans to aquire the shard once again. He planned to take the shard from Akar Kessel, but his powerwas too great while he possessed the Crystal Shard. They then made a deal that when Kessels life was over Ertuu would be given the shard. Ertuu agreed and then became leader of Kessel's armies. They gahtered orcs and giants alike to raid the Ten Towns.
This is where Ertuu first meets the dark elf Drizzt. Who outsmarts the demon and sends him back to the Abyss where he has to wait a century to take his avenge.