At first, the Demon Lord that came to understand what this identity of his meant, the role that he had to play, and the truth about his 'authority' couldn't come into terms with any of that.
He spent the first day in a pit of suicidal thoughts. On the second day, he washed these thoughts away by massacring all of the monsters that had crossed over to his former ruined world. On the third day, he wandered around the world during its last twilight moments before going back to the Demon world.
<Your Origin has been strengthened for accomplishing the duties of a Demon Lord!>
The desire to destroy all monsters writhed inside him, but even if he could destroy those that took part in the invasion, exterminating the inhabitants of the Demon world was virtually impossible even for someone like him.
Both the old and the current were called worlds, but the Demon world was in a whole other league. It was an endless realm that spawned monsters at a terrifying rate.