Servant of the Shard by R. A. Salvatore

This book brings two familiar characters into the limelight for the first time! Jarlaxle, a dark elf assassin, and Artemis Entreri, a human assassin, work together and against each other in Calimport. Their conflict is intensified by the influence of the Crystal Shard, a malevolent artifact that has been raousing trouble in the Forgotten Realms world since its introduction in The Crystal Shard.