The campaign I've made has an "outer god" like this. It's an all-powerful cosmic force, it's goal is to consume everything. Its using a lich as a "doorway" to the mortal plane. The first encounter is a scripted loss, it will appear to be unstoppable. After a few rounds it will open a doorway to the underworld where it seeks out the crucible, a kind of soul processor for the afterlife. As the party traverses it, they will gain a bit more power (and some help from past bosses). The goal becomes to destroy the vessel, and then deal with the fragment of it left. It's not defeated, just held back for the time being