MegaBug

The Megabug is the true main antagonist of Mario + Rabbids: Kingdom Battle. It is responsible for dumping the Rabbids in the Mushroom Kingdom and controls the entire species until they are defeated individually.

Akin to the Void from Super Paper Mario, it grows larger throughout the game and threatens to engulf both worlds.

In the final boss battle, it posesses Bowser who had recently returned from vacation and absorbs Spawny, taking the form of a phoenix-like being, but it is eventually defeated by Mario and the crew, saving Bowser, Spawny and the world.