Option One Or Two Or Both.

Legion hates making losses, but they hate wastage the most. They hate it so much that even if they don't need something, they have to exchange it for something useful to them. This plane has been ravaged, but that doesn't mean they will let it go. 

They don't need this plane for the era of conquest since they already have one in which they have invested heavily. Besides, they finally have the power of a half-world god at their disposal, so they certainly don't need this plane.

The smart thing is to turn it into something useful for them. So Aeternus and the other clones thought about what they could do to take advantage of the Gattling plane.

The first idea that came to them was selling it. A demon god won't be interested in a plane that has been looted of all life, including the will of the plane, but they can deceive the demon god by hiding the fact that they have already attacked this plane.