Right now, the Beggar King is only a rumor. His name has come up in connection with the reactivation of the charnel tower. He is also said to be associated with Mother Zeb’oltha (a dark warlock), and could possibly be contacting/summoning devils.
Arthuro the Fence has confirmed the existence of the Beggar King. The Fence mentioned him as the person who allows him to sell his goods, but Arthuro really doesn’t know much more about the Beggar King than any other rumor source.