The assassin was a very vicious and evil person. She would kill anyone for no other reason than being paid.
During a mission, Kueh-Ku acquired a strange black gem that somehow burrowed its way into her flesh, buried in the center of her left palm. This gem absorbed all magic cast against Kueh-Ku but also made her unable to use magic items. Nobody knew if the gem was a sort of ioun stone and would burn out or if it was a permanent power.
The half-elf worked as an assassin for the Shadow Thieves. Once during an assassination, she acquired her strange magic-absorbing gem. Initially, she tried to get it removed but soon found it very useful. Kueh-Ku was considered by the guild a key agent in their plots against the Lords of Waterdeep.