Probably either fighting Vesra or Terul depending on your actions taken throughout the game.
Now I know an idea like this would be completely unreasonable to actually do, but hear me out: What if all major decisions made throughout the game added either “Vesra Points” or “Terul Points” to you, which are these sort of “points” that you can’t really see, and instead you can only know which path you’re on based on what the Oracle tells to you (ex. “You seem to be following the path that Vesra has laid out for you” or “You seems to be following the path that Terul has laid out for you”)
Actions that could contribute to your Vesra/Terul Points are picking a subclass (which heavily affects your Vesra/Terul allegiance, except for if you pick a neutral one in which case no points are added/subtracted), killing enemies with certain weapons a lot (Terul’s Talon, Vesra’s Eviscerator, etc.) or doing other things in the future like praying in certain shrines to Terul or Vesra and stuff like that.
You can challenge the final boss when you reach a certain area which requires a certain level to get to, and based on if have more Vesra Points or Terul Points, you will fight the opposite god. To get there, though, you have to do something wild. Maybe like a raid full of extremely powerful, constantly spawning enemies which you have to survive for two hours, with short, 1 minute breaks between enemy waves every 10 minutes and a miniboss wave followed by a large break after an hour.
When you finally get to the two hour mark, you are pitted against an extremely difficult boss, and if you beat them then you get access to the final area. This area contains a shop to restock, and an NPC who will let you enter into a battle with Vesra or Terul.
No clue what would happen after, though.