DaemonNahash

This is so amazing! All power to ya @ShogunTaira; wishing you only the best in this endeavor!

Firstly, hats off to you; this premise sounds very exciting!

If I understand correctly, the ruler of the overworld is just that: the ruler of the overworld only, not the entire multiverse (hence the need to use the protagonist to get the multiverse under his control). This would mean that if he and the protagonist are at first on the same side, the armies of the overwatch world likely won't be the source of conflict in the first half, as they have the same/similar goal as the protagonist. It seems to me that the governments/principalities/armies/whatever of the other multiverses would have conflicting goals with the overwatch world and its ruler.

So if this protagonist is going to be the apprentice/mentee/poster boy of Mr. Multiversal-Emperor-to-be, you could spin it as the protagonist being mentored in the wrong direction, i.e. guided into conquering places that don't want to be conquered while thinking he is doing good. The second half could then involve him realizing his mistake and investigating the deception of his dear mentor, unraveling the lies to find a new (and true) antagonist. I actually wrote a story with a similar reversal, but that's by the way.

This is, of course, only a suggestion; feel free to pick, choose, and pass as you see fit. Best of luck!

How interesting...I shall be watching this with great interest