The Heir to Both Worlds (両界の嗣子 Ryōkai no Shishi) is a being created by the fusion of an existence from this world and an existence from the Crimson Realm. It was originally meant to be the existence born through the Grand Scheme advocated by Asiz. Hundreds of years later, Pheles and Johann, the "Engage Link", succeeded in creating "Heir to Both Worlds" Justus.
The Grand Scheme[]
The birth of the Heir to Both Worlds was the objective of Asiz's Grand Scheme. It was to be the being created by the fusion of the Crimson Lord Asiz and the Flame Haze he had been contracted to, "Weaver of Coffins" Tis, through the activation of the Unrestricted Spell Insignias of "Disintegration" (分解 Bunkai) and "Fixation" (定着 Teichaku), with the insignias being fragments of the Psalm of the Grand Order.
In the final stages of the Great War of the Middle Ages, Asiz got the Treasure Tool Nachtigall, which he had gained control of through the vast amount of Power of Existence he possessed, to use the two extremely difficult to activate insignias of the Psalm of the Grand Order, which were carved on a metal plate. Using the two insignias, the Heir to Both Worlds was to be born through the following process:
- Through the insignia which disintegrates existences, a part of both Asiz and Tis' existences were disintegrated into threads.
- Through the insignia which fixates existences, the parts of the two's existences which were disintegrated into threads were gathered to form a blue crystal.
- The two beings would then be fused into one.
The blue crystal which was to become the Heir to Both Worlds was destroyed before it could be born, along with Tis' corpse which had been preserved by Pure Coffin and the metal plate with the Psalm of the Grand Order carved on it, by Alastor, who had been Divine Summoned as the "God of Retribution" through Mathilde Saint-Omer's activation of Tenpa Jōsai.
Justus[]
Centuries later, in the Modern Era, the "Engage Link", Pheles and Johann, sought to interweave their existences using the Psalm of the Grand Order altered by Pheles as the core. The birth of the Heir to Both Worlds was successful, using a vast amount of Power of Existence Crimson Denizens had left in this world when they departed to the newly created New World Xanadu.
The "flow" of the heir was transient, such that it would disappear if left alone. Therefore, the "God of Guidance" Shaher, who caught sight of it through Rofocale, made the existence of the Heir to Both Worlds known to all Denizens.
"Heir to Both Worlds" Justus, the first of his kind to be born, could naturally move within a Fūzetsu. His rearing was undertaken by Wilhelmina Carmel and Tiamat, friends of the "Engage Link", and he was to be raised in the New World.