The Ruler

is a reversing power possessed by the Demon King.

Description
The Ruler is a magic ability that allows the Demon King to "reverse" all magical attacks and powers effects used against him. All attacks and weakness are converted into healing and fortification.

Even the effects of others' powers are completely reversed; per example, when Ban tried to use his Snatch to steal Demon King's strength, he was forced to give his own to the ruler.

However, as The Ruler functions on the concept of magic inversion rather than magic absorption, any magic meant to strengthen him or increase his power will instead weaken him, as shown by Ban. Despite this weakness, he can easily overcome it by deactivating his own power.

Trivia

 * Interestingly the Supreme Deity has an unnamed ability that has similar effects to Demon King's The Ruler in a way it would be considered the opposite of it, where instead of reversing magical attacks into healing, it reverses physical attacks.