Within the rift between realms, you've met a powerful Demon.
Assaulting him within his own realm will be difficult, but it's time he perished.
Attacking a demon within his own realm may have been a tad foolhardy. The demon was invulnerable, but you managed to smash an altar of black glass that seemed to contain the demon's essence. With it's destruction, the Prime Demon shattered into countless shards of stone.