So basically Celaena is assassin and will kill and/or torture whoever.
She can be hired by anyone to basically do anything like get revenge or information for anyone. She can also try to destroy alliances by killing npcs and making the scent of the murder someone from an Ally clan. Also, she can kill your character if you want him/her killed off.
As in the title she does this for free^^
So just state your name, your clan, and what you want for her to do and I'll be onto it!