As aeldredg said, he's Drizzt's nemesis. He's considered as Drizzt's equal opponent or human equivalent. Note that the last Drizzt book I've read is Passage to Dawn (which I have no finished yet), but Artemis Entreri is The cool villain in the Drizzt books. There's a description of him in the 2nd book of the Icewind Dale trilogy I believe and IIRC, it says that he has dark hair (possibly black), he's attractive (from Cattie-Brie's point of view anyway), but his eyes are really the only thing that distinguishes him; they're cold killer eyes that have lost all emotion.

He's the perfect assassin. In fact, he's one of the best. He was hired by Pasha Pook, but it was the crime boss of Calimport that wanted the gem that Regis had stole from him. Entreri was supposed to bring back the halfling and the gem to the crime boss.

His knife is able to inflict additional damage when it is thrusted upon an opponent. This was described by Regis when Entreri had captured him and tortured him with his knife. I believe there was a name for that knife as well.

He is considered as Drizzt's equivalent, because he's almost the only one that has come close in beating Drizzt. He posseses two weapons as Drizzt, but unlike Drizzt (which is ironic, because Drizzt is a Drow and it is his nature to be evil) Entreri posseses no morals at all. He kills without resentment. He is after all an assassin.