The Grey War was how a compiled series of events was called. It involved the Blackbird attacks prior to the Insurgence and the Insurgence itself.
It was named after the House of Grey since they couldn't use the Cinis name. About 0.0125% of the Underworld population perished.
The War[]
It was originally believed to be a series of battles fought by benders of the Court against a Northerner force led by James Saint-Vincent. What actually happened was that in the year leading to what eventually became known as the Insurgence, the Order of the Blackbirds led various attacks against Allegiance members.
The popular belief is that what started as a movement for parity lost its way as too many people got involved, which led to the Insurgency and the death of hundred of thousands of people. The truth is that what became the Insurgence was an attempt to open the portal to the North permanently, but as they didnt have all instruments, the recently opened portal grew unstable and exploded.
Fall of House of Sith[]
It was belived, James pursued all the members of the house around the world in a modern blitzkrieg fashion, articulating several attacks in apparently random locations of the globe, each being led by one of his officers, at once. What actually happened was that since the blood of (almost) all Siths -- which Lanna and Will colected in the decade leading to the event -- was presented in Will's self-sacrifice, they all died.
Aftermath[]
James was considered dead by the Court. The few people who survived the Insurgency -- Richard Moreisntein, Harinnah Jtuhier etc. were declared uninvolved in the final orchestration -- a lot to do with them being members of the Original Houses.
In spite of all the bloodshed, a last member of the Sith remained alive, Nate Sommerfold. He was put under the custody of the Court and being constantly watched giving he was found to be an Envisioner. Instead of killing him, James and Richard opted for using him for their profit and Richard, who already had a son, was voted as legal guardian of the child.