So I've been reading the "Red Queen's War" Trilogy by Mark Lawrence, whereby a supposed "Daughter of Loki" resides within the protagonist and talks to him at night. Alongside an Archfiend Patron I was running for one of my Warlock players in D&D, I've kinda been on a villain within/behind the scenes type bend recently. I say it's recent but really that was a huge reason for my infatuation with the story of The World Ends With You actually... So everyone look forward to that when the anime drops I guess! So here is what I've come up with based on my experience running the Patron-Warlock relationship in that concluded campaign:
The protagonist is a young person of short stature and good-heart. They belong to a small family of simple means, alchemists by trade, trying to stay afloat within a large city of guilds and artisans. Still they are always content and optimistic, driven to goodness through the family bond. Their elder sibling however, heir to the family business, is ambitious and seeks to create the ultimate alchemical elixir. Having found an ancient text of questionable repute that speaks of a Pit of Flames that burns with pleasure and purification enough to heal wounds and sustain life, the sibling set about experimenting with rituals and magics to attempt to bottle some of this miraculous fire. However, the Pit of Flames is in fact the volcanic centre of one of the layers of Hell; a place of pain and punishment as much as pleasure and purification, and only able to sustain and benefit the most powerful of fiends and devils. The rituals tore open a rift to this place, inside which resided a monstrous Pit Fiend, returned to the pit to heal from a mortal wound and in the process of metamorphosing into something greater. The Fiend threatens to bathe the house in hellfire, seeing as the sibling sought the fire in the first place, but the protagonist steps in and strikes a bargain with the Fiend, offering their servitude in exchange for no harm coming to their family and home. This is an attractive proposition for a Fiend, seeing as their status within the Hells is one that is won by strength of arms, and they must build an army from damned souls. The Fiend is unable to leave the Pit of Flames so having a proxy within the overworld will give them the hands required to corrupt and build whilst still recovering. The protagonist and the Pit Fiend form a pact, bestowing some of the proto-Archfiend's magic upon the person - a volatile magic of fire - and maintaining a sliver of the open rift in the mind of them too to provide a window for the Fiend to peer through. When the Fiend speaks through the window its voice is a rumble laced with wicked amusement, backed by a choir of spitting lava and crackling flame.
The protagonist, afraid of the wild and vengeful nature of the magic within them, leaves home to protect their family and does their best to act in accordance with their truly good nature - helping those in need and just generally being friendly. The Fiend, however, seeks to corrupt the protagonist, as well as lead to the corruption of others, e.g.: Resorting to minor acts of arson by flaring the magic within the protagonist to incriminate them and force them into conflict with other good people; or pretending to have insights into the motives of others, causing arguments and obfuscating the truth of matters; or reducing the protagonist's access to magic at critical moments, forcing them to make further promises in order to survive. On top of this, where cults to the Pit Fiend do exist, the protagonist is expected to stand aside and allow bloody rituals to be completed.
In one instance the protagonist came across a ritual where cultists and disguised lesser fiends in red robes were attempting to create a weapon befitting a general of an army of Hell. At the centre of the ritual, a captured paladin, surprisingly young, lay impaled by his own sword to a stone altar, still alive. Surrounding the altar, forming the points of a pentagram stood large metal cages filled with wailing devotees to the paladin's god. The ritual continues with chanting in an infernal tongue, and a red-robed figure lights a torch with a violent orange-red flame in the centre above the altar, ripping the paladin's soul from his body. They proceed to slowly walk out, in time to the chanting, to the first of the cages, and light a large brazier that stands behind the cage.The fire catches in the brazier, and a split second later the prisoners in the cage scream as they burst into a horrifying tangled conflagration that rises into a fiery orb above the spiked, black iron. The Red robed figure begins his slow, timed walk back to the centre, and passes the torch to another, who begins to turn towards the second cage. When the ritual completes, the soul-fueled fire will meet in the centre and bind the paladin's soul to the weapon where it will be tortured and corrupted whenever weilded. The instructions come through loud and clear in the back of the protagonist's mind, "Do not intervene". However, they have seen enough. In an act of direct defiance, they interrupt the ritual and free the prisoners. The incomplete ritual binds the soul to the weapon, but free of corruption - creating a weapon with the paladin's sentience. The Pit Fiend is enraged and punishes the protagonist but cannot sever the pact themselves.
Unknown to the protagonist is that a Fiend's True Name holds power over it. They know the fiend by the name it has provided but, should they discover its True Name, then they will control its power and be able to free themselves from the Pact.
So yeah, this was something really cool to play out in D&D and I think it's a quite cool story in itself :)