Shukun Shintan

Shukun Shintan (主君深潭 shukun shintan; Eng Lit Translation, "Lord of the Abyss,") is an entity who was assumed for a Millennium to have been merely a mythological horror character, for both the Spiritual and Living worlds. However, he emerges to be the oldest and most powerful Ikiryō (生き霊 ikiryou; Eng Lit Translation, "Vengeful Spirit,"), bordering upon the ascending class of predating Demons of the netherworld, allying with the arisen and ascended Sōsuke Aizen in his alliance with the group known as the Sennin Butai of like-minded near to perfect immortals.