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Demonic Contract, a rare drop from cadavers.
Is not a demon but it occupy a demon slot nonetheless, and when you summon it you either have 20% chance for it to permanently turn into a Unique demon or 80% chance for it to crumble to dust.
The thing is, it level up by staying in the inventory like other demons, so every time you level it up the chance for turn into a Unique the first time you summon it increase by 10%, so is kind of high risk high reward type of item, meaning that if you "seal away" one of your demon slots to carry this thing around as much as for 7 level ups, then you have 90% chance of getting a random Unique
Tought the idea was fun since impose a difficult choice on the player and possibly good or bad surprises (maybe also a % to actually summon something really bad), but can't really tell if gameplay wise something like this would play out nice, anyway thought was worth to get the idea out

Hmm... it sounds kind of similar to how the Recycle relic upgrades operates, in that it makes you "spend" stable slots on something for a time to possibly gain a benefit later. I might be able to work with this. I've added it to the idea pile. Thanks!

Well there already is. Just carry library of skills you might need to deal with situation and adapt. Dunno if it is useful but costs seem low enough so when your party is already built and you have nowhere money to spend with some money gain relic ugrades.
Using revelation or fusion thrals to fish abilities out of perk.