Brother Valmont was a sorcerer and a member of Constantine III's court.
Earlier in his life, Brother Valmont may have been a monk of some kind. Eventually, he left his order and went on to pursue other, more nefarious goals. 
In 997, Brother Valmont accompanied King Constantine on the hunt for Mary and Finella as well as joining him on his campaign to destroy every gargoyle in Scotland. However, Valmont's true plan was to gain possession of the Grimorum Arcanorum and he had no true loyalty to Constantine. ("The Gate")
During the Battle of Rathveramoen, Valmont seized the Grimorum and used its power to rain down a storm of magical arrows killing countless gargoyles and humans in both armies (Magus the horse perished as well). His plan was to kill both Kenneth III and Constantine and bring Scotland to its knees. ("Tyrants")
Valmont was foiled when Brooklyn severed his hand, which had been holding the book. The sorcerer fled.
The full extent of Valmont's plans remains unknown, save that he desired to possess the Grimorum Arcanorum, the Phoenix Gate, and the Eye of Odin. He also referred to the book as "the pathway to Avalon". ("Phoenix")