I have a bit of a headcanon for a warlock character in that he is his own patron, not that he knows this. The idea being that, born as a tiefling and have grown up being subjected to the prejudices against Tieflings all his life, he begins to believe that he is fated for Hell regardless, so he decides to embrace it. In a self-fulfilling prophecy, he comes in contact with a Devil (those are the lawful evil ones yes?) who makes him promises, and he willfully submits. When he becomes convinced that he would become a Lemure, he embraces his nature even more fully to try and 'work off' this fate and to be promoted, upon his death, to a higher level devil and skip the basic level.
That's a great character concept, and there's actually precedent for this. In the recent 5E adventure Descent into Avernus, player character might end up striking various bargains with the rulers of the Nine Hells. One such bargain has the player character's agreeing to have their soul, should they die after achieving 17th level or higher, sucked into the 9th level of Baator and turned directly into a Pit Fiend (the highest level of devil) to serve as a general in Asmodeus's army. So definitely possible.
