The Proving Massacre was an event that involved Nora brave candidates and Eclipse soldiers. The massacre functioned as the catalyst for events in Horizon: Zero Dawn.
Events Before the Massacre
The Nora have held the tradition of having their Brave candidates take part of a traditional test to see if they have the opportunity to be Braves. Among the new candidates was Aloy, an outcast of the Nora who had come to get answer over her mother. There she met Olin, who at the time began to act oddly, but Aloy saw it as nothing of importance.
Unbeknownst to the Nora, HADES noticed Aloy through Olin's Focus and issued an order to Helis and the Eclipse to kill the girl.
The Proving
The brave candidates were to climb All-Mother's mountain, take pieces of Grazers through an obstacle course and whoever made it first were considered braves and speak to the High Matriarchs. Despite having difficulty with Bast over a Grazer piece, Aloy took the piece and managed to cross a disused obstacle course and achieve victory as the first brave, which meant that she could speak to the matriarchs over the issue of her mother.
The Massacre
Aloy witnessing Nora murdered by the Eclypse
With Aloy the victor, Bast tried to challenge her victory, but they were soon interrupted by a hail of arrows that struck down most of the other candidates and the proctor. Aloy, Bast, and Vala tried their best to fight back the attackers, but Bast and Vala were killed by a gunner and Aloy was left unconscious.
Soon, one attacker grabbed Aloy and, holding her from the neck over the cliff edge, attempted to kill her, but was interrupted by Rost. He defends her as best he can, but soon takes note of a bomb in the area, and saves Aloy by pushing her off the ledge and sacrificing himself in the ensuing explosion.
Aftermath
The Nora, as well as many visiting members of the other tribes, were shocked at the scope of the massacre, and were surprised that the sole survivor is Aloy. Those responsible of the massacre are soon discovered to be Shadow Carja, The Matriarchs send a brave party to hunt down the murderers, but they themselves are killed when Corrupted Machines ambush and kill most of the braves. Soon after, Aloy recovers and is sent to discover and stop the problems that have risen because of the massacre.