The ROOD

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.

Principles
The letters (short in Latin for Iēsus Nazarēnus, Rēx Iūdaeōrum) are believed to have been carved into the original cross.

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

Coronzon Arc
During his final battle with Aleister Crowley, in response to his opponent using the linked to the Son of God to build his body up to a higher level, the reproduced Mathers used a method based around INRI, the holy letters carved into the original cross, to purify and enhance his body in a similar fashion.