The Trinity (三位一体 Sanmittai) is a group of 4 Index apostles: Sebastianus of the Holy Bone, Irene of the Dragon Skeleton, Benedictus of the Bookshelf and Scholastica of the Bookshelf. The main purpose for the combination is that Sebastianus is an expert at close quarter combat who will attack head-on, Irene, who has the Dragon Skeleton will attack the enemy from a mid-range and then Benedictus and Scholastica will support from the rear with their magic. Together, they make a powerful combination and was recognized by Index.

In 1945, Index sent the Trinity to Japan under the guidance of Georgius of the Rainbow, to be a part of the Punishment Squad, to defeat Lieselotte Werckmeister, believing that the team, under the leadership of the strongest Saint, would enable them to achieve their goal. However, Lieselotte proved to be too powerful and the Trinity was defeated. After the battle was ended by Georgius, the Trinity was resurrected as the three Black Knights: Ira, Invidia and Acedia (Benedictus and Scholastica shared the same body).

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