The Haereticum, meaning the "library of blasphemies", existed in the Shadow World as a virtual construct. Like the rest of the Shadow World, it was created by the Monks, so that they could plan the perfect invasion of Earth.
It was founded by Pope Benedict IX in the 11th century. It was a secret underground library in Vatican City which contained the text Veritas, written in a language only a few could understand. The library contained secrets so great that some committed suicide after discovering them. (TV: Extremis)