The world is a Dungeon. The Dungeon Core shattered long years ago in an event called the Splintering. The shards of the Core scattered across the world. The large ones formed mini Dungeons of their own. The smaller ones turned into Skill Shards.
The inhabitants of the world can fuse with Skill Shards to get skills. The shards can fuse into myriad Dungeons and mages at the pinnacle of magic have their own personal Dungeons. They are the Dungeon Mages.
Our MC travels back in time from the future and uses his knowledge to free humanity from the oppression of the other races.