The Overflow world is a world in between dimensions and acts as a disposal site for worlds.

Anything can end up here from actual junk to random lost items and even chunks of land, even other living beings can find themselves here.

The inhabitants comb this world as they make use of what they can find especially living beings that don't live in this world, those that get caught are turned vassals or slaves depending on who catches them.

The Slave Traders are specialised in finding and catching foreign beings and they train them to be slaves before selling them.

Damian is a regular human-looking member of the race known as Umbra, but he has a secret hidden in his eyes.
Long ago when he was a baby a powerful demon was sealed in his eyes, this demon threatened to destroy the Overflow world which in turn will cause a chain reaction destroying all other worlds that connect to it when they dump their stuff.

Damian has inherited his father's castle and has gone to the Slave Traders to find his first vassal, once acquired he will start his journey to build up his forces to stop those that want to release the demon that is sealed inside of him.

I kinda got the idea for this world based on the game Trinity Universe
Parts of this are gonna be similar to Disgaea relating to mana spending for vassals.