Perhaps the single defining event in the history of the Order of Hermes since its foundation, the Schism War wiped House Diedne from the Order and had substantial ramifications for many of the Order's institutions and organizations, up to and including the adoption of Certamen for the resolution of disputes of a certain magnitude between members of the Order.
The conflict escalated slowly, growing mainly out of mistrust from the Latinate House of Tremere towards the Druidic House of Diedne, who were accused of practicing human sacrifice. The Schism War happened in early eleventh century. It caused the cancellation of many of the Regional Tribunals of 1004 and reached a point of no return when House Tremere declared war on House Diedne in 1010, soon to be followed by Houses Flambeau and Jerbiton.
Magi Trianomae of House Bonisagus then convinced the Primus of House Guernicus to call for an emergency Grand Tribunal of resolution instead of the Regular Tribunal that would otherwise happen in 1011. The Primi of Houses Bonisagus and Mercere quickly agreed, and the Grand Tribunal eventually decided by a close vote to renounce House Diedne entirely.
The Schism War was terribly divisible and remains a haunting memory to this day.