Grimgroth is a vampire of clan Tremere, and a member of the Tremere Inner Council of Seven. He is responsible for Western Europe including the British Isles and Australia. In life, he was a House Tremere magus of Mistridge covenant, which he betrayed.
Grimgroth is not one of the original members of the Council of Seven, having been elevated to replace the rogue Goratrix; this promotion was sponsored by Grimgroth's own sire, the Tremere founder himself. He habitually changes the location of his chantry very often, but at the moment his prime chantry is in Rome
During the dark ages the tremeres, allied with Éon de l'Etoile, take advantage of the Albigensian crusades to discover the foundations of mages hidden in the valleys and eliminate some cells of Salubrians on the way. This leads to the discovery and destruction of the mighty foundation of Mistridge belonging to the Order of Hermes, thanks to the betrayal of Mage Grimgroth, embraced by Tremere himself as a result of this coup.
Like all Tremere Councilors, Grimgroth is responsible for furthering his Clan's interests in a broad geographical area. Originally, his territory was France. In modern nights, he has been given responsibility for the entirety of Western Europe, including the British Isles and Australia.
Many rumors surround this figure, some say he cooperates with the mages of Rome to bring some sinister fate for all Europe, other came up with the possibility of Sabbat ties, to an extent that even implicate him as a spy for Goratrix. However, few facts are known for sure.
Mage: The Ascension and Dark Ages: Mage
Through the book Blood Treachery, it could be implied that Grimgroth was the magus known as the "Former Tremere" that was present at the fall of Mistridge in 1210 CE. Mistridge's demise can be directly attributed to him: he briefly lowered the Covenant's magical defenses, leaving the wizards of Mistridge vulnerable to the Craftmasons' cannons.
This hypothesis is confirmed in Dark Ages: Mage Grimoire when it tells us that, in 1210, Mistridge's defenses lasted until Grimgroth bani Tremere removed all Quintessence from the wards that protected the chantry (and that fact alone allowed the Craftmasons to emerge victorious from that battle).
It is unclear if this act was a deliberate betrayal or a momentary lapse of judgment, although it is certain that Grimgroth ultimately betrayed the Order of Hermes by joining with the vampiric Tremere as the original Massasa War was peaking.
In the modern era, Grimgroth led a counter-attack of the Tremere against a Hermetic chantry in London at the behest of an unidentified presence (probably the Tremere Founder himself), during the Second Massasa War. As he confronted the Hermetics over the reignition of the conflict, he warned them against using the blood of vampires and urged the mages (among them Rebecca Mitsokata bani Fortunae, Aurelien Archambeau, Josiah Lamb, and Edward Gilmore) to save the Order from itself.
In Ars Magica 2nd Edition, Grimgroth is instead said to be a member of House Jerbiton. His master is the magus Consuelia, and one of his apprentices is named Lucienne. In 3rd Edition, he belongs to House Tremere instead. It might be supposed that he began as a Jerbiton magus but later switched to House Tremere.
In Mage: the Ascension 2nd Edition, Grimgroth is again a Jerbiton, identified as the traitor of Mistridge, and listed as actually dead.