Emperor of the Four Realms

In the ancient era, a powerful human cultivator took advantage of the great versatility of human cultivation. He united the cultivation methods of humans, gods, demons, and monsters, ascended to true godhood, and suppressed all four realms in one fell swoop. From then on, humanity ruled the four realms. In the present era, the four realms are sundered and war has erupted. Who can master the arts of all four races, reunite the realms, and rule them once more?