Orb

"... Elaborate," Terry said.

"The worlds are full of mana, and those who rule it can use the mana as they see fit, the smart way is to create monsters and make them invade other worlds," the creature explained. "Once that happens, the mana of the said world will change and the overlord will be able to reach the next one."

"How can one become an overlord?" Terry asked. "Also, why can't overlords pass through portals freely?" 

"... First, you need to learn a certain spell to bind the world to you and the people in it," The creature said after a few moments of silence. "I can teach you that, it won't take more than one week to learn."

"Is that so?" Terry asked, and then he swung down the sword.

The sphere and the monster were split in two, and the creature didn't have time to react. At the same time, Terry felt his sword leveling up a lot… there was no point in talking with an enemy further.