The Old One is the living core of Tarrod.
Midian is a dark fantasy game. Most churches in Midian are moral belief structures & guidelines rather than a system that contains a deific figurehead (or many such) and an afterlife.
Context: The gods of Zetacron are physically present in the world. You can go and meet them if you like. Additionally, they are very human, with flaws and quirks—possibly more than regular people.
When the Gods forged Creation they imbued everything with a spirit of its own, a soul so to speak. The trees, the bushes, serpents and birds, everything was imbued with a soul. Even the very earth and the mountains got spirits of their own, as did the winds, the rain and the storms.
Angels are the guardians of Order, the forces that oversee that the purpose of existence is indeed fulfilled.
Demons are the darker emotions, the destructive forces of creation. In primeval times the demons awoke alongside the gods and the angels, and were herded by the greater powers, chased into hell, even as the angels were led into heaven.
God has been slayed by a mortal man and his head is now worshipped and put on a pedestal. The PCs are hired to steal the head and bring it to a mystic that claims to have the power to revive God.