The ROOD

The_ROOD (ゴルゴダの十字架 (The_ROOD) ) or True Cross refers to the physical remnants which, by a Christian tradition, are believed to be from the cross upon which Jesus was crucified.

Etymology
The author uses the word "The ROOD" in English to refer to the artifact in the original light novel. Today it refers to the cross or crucifix that are at the entrance to the choir or chancel of a church. However, Rood was originally the only Old English word for the instrument of Jesus Christ's death.

Index Arc
Index cites to Kamijou Touma the Rood, among other things such as the Spear of Longinus and Joseph's Holy Grail as the limits on how magicians are capable creating substitutes for ceremonies using modern materials.