Seeking Peace with a Demonic Heart

The story takes place 300 years after the Demon Emperor joined forces with powerful figures from the various races to quell the storm beyond the realms. Rebel forces in the Demon Clan plot to revive their people and unify the world, so they launch an uprising, kill the young lord of the Demon Clan, and dig out the Demon Heart, which represents imperial authority and power. But due to a botched handover between the rebel leaders and their troops, the Demon Heart ends up in the mortal world, escorted by a faction of the Demon Clan rebels...