Monster Guild: The Dark Lord's (No-Good) Comeback!

The Demon King has had a rough day. Just when he thought he finally defeated the hero, he came back. And back. And back. And sooner or later, the hero who constantly resurrected was bound to defeat a Demon Lord who couldn't. And he did. The hero vanquished the Demon Lord, peace spread across the land, and so on and so forth. The end. That's it. That's the whole story. ... Kidding. The Demon Lord sent his soul to a empty vessel, which saved his life but unfortunately also is incredibly weak. So what's a Demon King to do when he wants to rule the world, but couldn't even defeat a child?