De Vermis Mysteriis

Background
De Vermis Mysteriis (English translation Mysteries of the Worm) is a powerful book of spells that was written in the Middle Ages by an alchemist named Ludwig Prinn.

Prinn claimed to have learned the secrets he wrote about in the book during his captivity during the Crusades in 1271. However, there is no real evidence to back up this claim.

The Old Ones
What is known about De Vermis Mysteriis is that, like the Necronomicon, it can be used to free the Old Ones from their empty dimension prison. For that reason, like the Necronomicon, copies of De Vermis Mysteriis were destroyed over the centuries.

Rex Buckland's copy
One of the few remaining copies of De Vermis Mysteriis is among the collection of rare books and scrolls of the former Warlock known as Rex Buckland (said collection also includes a copy of the Necronomicon). Just how and when Rex obtained this copy is unknown at this time.

In 2008, in order to gain the trust of the Charmed Ones, Rex revealed his copy to them. He felt that, if they knew he had such books, but did not use them for evil purposes, that they could now trust him. And he was right, this helped convinced the Halliwell sisters that Rex and Hannah Webster were no longer evil.

In 2015, Rex had been engaged in translating his copy of De Vermis Mysteriis when he and Hannah became involved with the degenerate Martense family.

The Martense family's copy
As it turned out, the Martense family also had a copy of De Vermis Mysteriis which their ancestors had brought over to the New World with them. The family matriarch, Johanna Martense, claimed that De Vermis Mysteriis had told them to breed among themselves, and to prepare the way for the return of the Old Ones. She went on to say that the Martense family would then be greatly rewarded when the Old Ones did return.

This copy of De Vermis Mysteriis was destroyed when Rex and Hannah were forced to destroy the degenerate Martense family soon after (although Johanna Martense herself survived).

Other copies
Whether any other copies of De Vermis Mysteriis still exist remains a mystery at this time.