Note: You have to read the synopsis.

There are 2 types of Magic:

Destruction Magic[The Magic created by the Demon King]

Divine Magic[The Magic created by Deities]

The reason that the Demon King is so strong is that he was made with Divine Magic, but he made Destruction Magic, therefore making his magic an amalgamation of the 2 Magic types. Humans are made of Divine Magic and can use it but they lost all knowledge of Magic millennia ago.

There are 2 Worlds:

The Deties's World [Most Deities are deceased because of the Demon King killing them but all high-grade ones are alive.]

The Material World [The Universe and Earth.]