Great Old Ones

The Great Old Ones (, Gurēto Ōrudo Wanzu; lit. "Ancient Eldritch Horrors of the Pitch-Black Nether Regions") is one of the three known Black Arts forms created by Zetsubō Usuguraiboshi, said to be manifestation of his various negative emotions; it's part of the Megiddo Triad (, Megido Toraiado; lit. "Underworld's Three Demonic Superpowers"), a set of magics that's based on different aspects of Megiddo, the darkest region in all of the Heroic Chronicles series — heralded and feared as "the source of all known evils" by various beings. Due to this, this makes the magic one of the most dreadful of its kind overall, something its shares with its other two brethren.

Alternatively known as Demon Manifestation Magic (悪魔の顕現魔法, Akuma no Kengan Mahō), it's a Caster Magic and a form of Subspecies Magic — derived from the effects of Darkness Magic animated through the machinations of Living Magic and Enchantment respectively, bring about eldritch creations far surpassing the effects of Molding Magic and rivaling Arc of Embodiment in terms of realism. As both names oh so subtly suggests, this magic allows Zetsubō to conjure various demonic entities from the abyss using the powers of the vast darkness he skillfully commands, effectively summoning them in droves should he desire. It's due to this he's counted as one the few that's a literal "one-man army", conquering his enemies through sheer number alone.

Trivia

 * If one could tell, this is based on the infamous pantheon of the same name from the, one of the most terrifying universes to exist in fiction to-date. Most of the associated spells in this magic makes reference to that, glorifying the creator to such extremes.
 * The creation of Demons and evil entities has been done in other series, varying in designs; in this case, some of them are derived from Magi: The Kingdom of Magic, Mahou Sensei Negima!, and 11Eyes respectively — as they make extensive usage of such a concept to further emphasize their respective plots.
 * In a sense, this was slightly inspired by the Ea article, created by the author's friend, HB — one of the most well-known horror authors on the site.