Most human pantheons have one god that just doesn’t fit in. Adu-Nunna was the dark one of his pantheon, a god of incredible cruelty who used his followers, the wealthy and powerful, to take his hatred out on the children of his fellow gods. His Adunic Knights waged terrible warfare, executing non-believers and erasing entire nations, tribes and humanoid races from the planet.
His brothers and sisters would not stand for the cruelty of Adu-Nunna and his followers. In a rare feat of cooperation, they combined their knowledge and power to curse him to seven lifetimes as a human under the boot of his followers, lifetimes he would remember in every detail for eternity. He was a slave, a beggar, a trusted advisor wrongly accused, an orphan, an enemy’s widow, a prisoner, and an experiment.
Adu-Nunna came out of this a changed god. Weeping as he knelt before his fellow gods, he apologized to them chanting a mantra of repentance for eight times eighty million repetitions. He was offered a new name and new form, but refused. Forswearing the heavens, he took a mortal form, calling himself Adu-Nunna the Father of Mercy. He hopes to peacefully undo the empire of suffering he previously created.
Adu-Nunna’s priests and paladins are not allowed to directly kill anyone human, humanoid or intelligent enough to speak to humans. They may fight to subdue, although in some cases, permanent injury is permitted if it prevents someone from harming others or furthering the cause of Adu-Nunna the No-longer (the name his followers call his previous self). Taking the arm of an executioner, for example.
His Brothers of Mercy and Sisters of Insight (his clerics) cast spells as if they were two levels higher and healing spells always heal maximum. (This is to balance out the no-killing prohibition). His clerics strictly follow the no-sharp weapons prohibition, although they may use or carry daggers or swords for ceremonial purposes. His paladins may only kill armed Adunic Knights who refuse mercy.
Despite his regained immortality (however your campaign may define it) Adu-Nunna appears as an old man, scarred and crippled. He casts any clerical spell as a 20th level priest. He has established a church and a following of a few thousand. They remain in hiding. His new symbol is a teardrop against a black background. He does not hesitate to perform miracles in order to convert followers of his former self.
Of course, the Adunic Knights and his previous church and followers believe this story is heresy, invented by Gal-vani the Tree of Tricksters. They refer to “the cursed imposter” and “his weaklings.” His priests still receive spells as they did before, which they take as proof that their god is still with them. A few believe the story of his rebirth, but think it is a deception of Adu-Nunna himself to trap the faithless. A select few high priests believe the story and now seek to destroy the new version of their god. They do not speak of it openly and have various theories of who is granting their spells now. . Adu-Nunna himself is not certain why. Perhaps he exists in two forms at once? Perhaps another agent of evil has taken his old name.
This piece is meant to be a starting point. It is one of many posts about gods and religions you can use for your campaign.