"Warlock's Trial"

Just the premise to a plotline, very unbaked.

You are a penis owner. The demon offers you a pact. You have to wear a chastity device, but while you wear it, you develop magical powers that grow in strength. Once you take off the chastity, the contract ends and your powers stop developing. You can freely reinstate the contract by putting it back on. If you do this, however, your magic won't continue developing until you have surpassed your own personal record. If you are unbearable horny, the demon will happily fuck you, but on the demon's own terms. If you request sex from the demon, he won't let you escape until he is satisfied. Don't worry though. The demon is very skilled and very thorough, and you WILL be satisfied as well, if a bit tired, wobbly, and sweaty.

Variations can come into place here. Maybe it's a trick and the chastity device is impossible to take off. Trade the chastity for some other form of humiiliation/emmasculation such as being inflicted with incontinence and having to wear diapers, or being turned into a cuntboy (as a lover of cuntboys, this seems like a weak punishment). Trade the payoff for something else, like money or eternal youth. 

Maybe there's a whole group of fellow powerhungry warlocks taking part in this chastity trial and you all are competing to become the strongest. Maybe the nonstop chastity and sexy hijinks cause the whole thing to devolve into a submissive bdsm corruption/brainwashing plot.

Maybe sex with the demon involves being subjected to whatever kink the demon has on his mind at the time. Maybe you are tied up and subjected to various forms of BDSM. Maybe the demon transforms one or both of you into ferals or something. Maybe you are age regressed. Maybe you are then stuck as an animal/child for the next few days.