In Black Paint You Become Black. Volume 1

The Throne of the Demon Lord has been empty for a very long time, but the fragile balance is not eternal. The four viceroys of the demon kingdom have become strong enough to claim their rights to it. However, all four are approximately equally strong to win in an honest fight, and for an alliance, trust is needed, which is alien to these beings.

That's why the four demons came up with the most thrilling bet of the century: whoever defeats the strongest spellcaster of the era and absorbs his golden core will become the Demon Lord. It's just a pity that they forgot to inform the spellcaster about this, otherwise he is unlikely to agree...