The Montmartre Pact was the treaty that officially ended the hostility between the Gangrel and Nosferatu Clans and the Tremere in exchange for the destruction of the Gargoyle Creation Rite prior to the formation of the Camarilla.
In 1489, two members of the Council of Seven, Calderon and Meerlinda, met with representatives of Hardestadt's Camarilla in the French village of Montmartre. To appease those Clans they had offended in the Omen War, they vowed to never again create Gargoyles, releasing all under their control and ceasing to hunt those that had fled their rule. In exchange, the Gangrel and Nosferatu would abide any hostilities and the Tremere would be allowed to join the Camarilla.
While the rite remains in the archives of the Clan, usage is forbidden. Any Tremere caught in the creation of a Gargoyle will face expulsion from the sect as well as the Clan and given to the mercy of any Gangrel or Nosferatu. However, some Gargoyles have remained under Tremere control, either of their own, free decision or because the Tremere went at great lengths to keep their existence secret, using them only to guard their most treasured chantries.
Montmartre is a neighborhood within Paris, the city holding the Chantry of Goratrix, where that warlock experimented on and created many of the Gargoyles. The fact that Goratrix wasn't present, while his rival Calderon was, may be important.
- Rites of the Blood, p.105