DM Needs an idea to use Gargauth to add to a players backstory
Okay so the title is a bit strange i know but i have a blood hunter player that has a flushed out backstory already, hes in the middle of his character quest but he unfortunately died. now this player made a deal with Gargauth to give him the powers of the shield in exchange for freeing him back in candlekeep, if he fails he takes his soul.
The player came to me and asked that we find a way to bring his character back as he really wants to finish the quest so we decided that because Gargauth has his soul he will resurrect him in another body, but now his Pact of the Lycan bloodhunter will be a Pact of the demon slayer with a few more disadvantages to make things fair. like Gargauth being able to take control of his body 3 times whenever he wants.
now i know Gargauth is only a pitfiend so obviously i will change him to be a sealed away archdeevil or something like that.
I'm asking for any ideas to add to his backstory that will push him to now free Gargauth, like going to his old seat of power and having to fight the devil who helped the humans seal him or something like that.