The First Hellonic Blade
Cover art is Relevant
Aquarium is The Lord of the Waters, a man capable of using a demons very strength against it. In his home world he destroyed, with his brothers, he ruled. After destroying it and using the collective powers to bring him here, he was the only to not be reduced to a simple form. His brothers, until he gains enough demonic powers, are reduced to mere gems that are on the scabbard of his weapon.
Equestria, unbeknownst to most, is riddled with demons most of a minor status. After nearly fourteen centuries of collecting the demonic powers, he has learned of the return of the most famed demon of all in Equestria.
That one that dared possess Princess Luna.
The Nightmare Moon