The First Betrayal of the Jiralhanae took place during the events of the story Vadam Legacies: The Darkest Hour. Although it is one of the key precedents to the Covenant schism, it had been covered up and left unknown to the majority of the Covenant.
The event began on September 17th, when Tartarus and a platoon of Jiralhanae arrived at at the mobile military city Stoic Courier, and assassinated many Sangheili recruits there. Autel 'Vadam and Fira 'Demal managed to warn Field Master Bata 'Noromee and the Sangheili present were alerted to Tartarus' intents. The Chieftain left Stoic Courier, but one of his captains, Cerberus, remained behind.
When Autel made it to High Charity with a group of Sangheili, they were imprisoned by the Prophet of Regret, who had planned the betrayal all along after leaked information that one of the Halo rings were discovered. The Sangheili escaped and after forming plans with Sesa 'Refumee, they headed to Threshold to warn Supreme Commander Thel 'Vadamee and brought back reinforcements. The Sangheili then launched an assault on cruiser CCS-U572 and took over key parts of the ship. However, they had to leave when it was forced into Slipstream space due to a high-priority crisis.
At Stoic Courier, Fira was discreetly rescuing captive Sangheili recruits and bringing them to officer 'Refumee's corvette. The operation was halted when previously mentioned crisis occured, forcing the city into Slipstream space. Unfortunately, Fira had already overloaded the reactor to destroy the city, and they barely managed to take it back into normal space and escape before it was destroyed.
The conflict ended when Installation 04 was destroyed, forcing the Covenant to become one again. The relatively few witnesses were kept the event secret due to higher issues. The Prophet of Regret had also left High Charity to find the Ark and was later killed by SPARTAN-117, resolving any disputes from witnesses.