Another case of /tg/ discussions inspiring something interesting. The original subject was an order of knights that fought demons by pairing themselves with demon-infused items. No details were given beyond this; one of the subsequent ideas that quickly came up was that this was actually a way for demons to gain sanctuary, since Hell isn't exactly a fun place for them to be in either. My reaction was to expand on the idea using cues from the Soul Eater anime/manga series (whose protagonists wield partners that can turn into weapons).
The ritual itself is simple; you put out the call, and a demon who chooses to answer you is summoned. If willing, said demon can then be converted into a weapon, while retaining their consciousness and an awareness of their surroundings (including an ability to sense other demonic presences). The weapons they form negate virtually all forms of damage reduction and regeneration (or the setting's equivalent), making them great for monster hunting. In addition, they enhance the wielder's physical prowess (speed, grace and/or strength, distribution varying with each weapon) to a degree that corresponds to the bond between demon and wielder.
This bond is achieved through intimate mental interaction- I'm not saying the process is somehow sexual, there's just nothing more 'intimate' than directly interfacing with someone else's head. It would be both melodramatic and a terrible understatement to call this a 'battle of wills'- two vastly different minds must reconcile their differences, with each picking apart the flaws that lurk in the blind spots of the other's personality/ worldview, a process that no personality can escape wholly unscathed. Every demon that undergoes this process is humanized to some degree, and every human gains a new outlook on life that has a decidedly ruthless edge.
In the end, to have derive power from their bond, a wielder and weaponized demon must trust each other. They must reach some kind of understanding, one where each has no quarrel with the other, nor hides any secrets beyond those the other knows of and allows their partner to keep.
The tricky thing is that not all demons who participate in this process seek redemption. Yes, any demon who participates in such an arrangement is branded as a traitor throughout Hell, an outcast any other demon may freely rend to pieces. But if they gain enough power through the bond- a bond which tolerates no deception, but can be used to twist a mortal to their way of thinking. . .