Devourer (Reality Benders Book #8): LitRPG Series

The aggressive invaders from another galaxy cannot be stopped. The Composite keeps capturing system after system wiping out everything in its path, and there is no sign of a force capable of standing up to it.

In the search for allies, Gnat embarks on a long journey aimed at uniting the various branches of the human race. And though the potential for alliance is there, how can he convince them to set off to fight a suicidal war with such a disastrous balance of forces?

Beyond that, things aren’t as easy as they initially seemed with the humans from the far-off Empire, and they have a compelling reason not to enter this war which does not concern them. There's also the status of sole ruler of humanity at stake, which is not the kind of achievement that can be shared.

And that means one of the human leaders will have to back down.