The Promise of the Witch King is book 2 of the Forgotten Realms trilogy Sellswords. Book 1 was originally part of the Drizzt line of novels and has now been re-released with some short stories making up for the gap in time.
This book contains the adventures of Artemis Entreri the assassin and Jarlaxle the Drow, formerly of House Baenre, with two new companions: The ghost of a powerful lich called the Witch-King and an oath-bound Knight.