He asked the friends for dark magic in exchange for the souls of those who would die in his worldly conquests. In return, Chernabog pointed Merlock towards the duck genie's lamp. The amulet was a present from the friends. When Merlock died, The Friends decided to let him have one last chance by killing Donald and Daisy, only to be killed again. He's kept around when there's a need for an Evil Sorcerer the higher-ups are too busy for, just as long as said missions don't involve the ducks.