The Proving Massacre was an event that involved Nora brave candidates and Eclipse soldiers. The massacre functioned as the catalyst for the events in Horizon Zero Dawn.
Events
Before the Massacre
The Nora have held the tradition of having their adolescents take part of a traditional test to see if they are worthy of the opportunity to be Braves. Among the new candidates was Aloy, an outcast of the Nora who had come to get answers as to 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 to the finish line first were considered Braves and allowed to speak to the High Matriarchs. Despite having difficulty with Bast over a Grazer piece, Aloy took another Grazer part, managed to cross a disused obstacle course, and achieved victory as the first brave, which meant that she could speak to the Matriarchs about the issue of her mother.
The Massacre
Aloy witnessing Nora being murdered by the Eclipse
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 off the attackers, but a cultist killed Bast and Vala with a machine gun, and Aloy was left unconscious.
Soon, one attacker grabbed Aloy and, holding her by the neck over the edge of the cliff, attempted to kill her, but was interrupted by Rost. He defended her as best he could, but soon noticed a bomb in the area, and saved 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 was Aloy. Those responsible for the massacre were soon discovered to be Shadow Carja. The Matriarchs send a party of Braves to hunt down the murderers, but they were killed when Corrupted Machines ambushed and killed most of them. Only a few Braves survived the ambush, one of them Varl. Soon after, Aloy recovered and was sent to discover and stop the problems that had arisen because of the massacre.