Valefor is the greatest thief in the Underworld. This duke is responsible for the good relations among thieves. He's like a glue that keeps these guilds from falling apart. I don't know where the description of the lion with the donkey’s head that we see in the Lemegeton and Pseudomonarchia Daemonum comes from. There is indeed the story of Abdel Aman I which I will relate later, but the trail is thin.
In his delirium, he describes him as a giant monster, hidden under a cloak, with thousands of tentacles. Like the famous Ageless King, you can become the best of thieves by promising your soul to him.