The Proving Massacre was an event involving Nora Brave candidates and Eclipse soldiers that occurred during the Proving of 3040. The massacre functioned as the catalyst for the events of Horizon Zero Dawn.
Before the Massacre
The Nora hold the tradition of having their adolescents take part in a traditional test to see if they are worthy of the opportunity to be Braves. Among the new candidates was Aloy, an outcast of 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. Under Helis' orders, they were to avoid contact with the Nora if possible, but kill any witnesses.
The Brave candidates were to climb All-Mother 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.
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 held off the attackers long enough for a few of the other Braves to escape. While a few did so, a cultist killed Bast and Vala with a machine gun. Aloy then managed to kill off most of the remaining cultists on her own.
Soon, Helis 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.
The Nora, as well as many visiting members of the other tribes, were shocked at the scope of the massacre. 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 north of Mother's Rise. Only a few Braves survived the ambush, one of them being Varl. Soon after, Aloy recovered and was sent to investigate and stop the problems that had arisen because of the massacre.