The Gi Invasion is a war in Final Fantasy VII that occurs in εγλ 1959, in which the war-hungry Gi tribe attempts to invade Cosmo Canyon. The conflict is largely forgotten by the time of the events in Final Fantasy VII, with Bugenhagen being the only one who appears to know about it.
Though nearly extinct during the events in Final Fantasy VII, by the the time, most of the inhabitants of Cosmo Canyon were the lion-like species Red XIII belongs to. They serve as the guardians of the peaceful Cosmo Canyon.
The malicious Gi tribe lay siege on Cosmo Canyon, and attempts to break through by attacking through the Gi Cave. Seto, the only guardian of Cosmo Canyon, rushes to the cave to face the attackers alone. He singlehandedly holds off the invasion, and keeps fighting even after he is hit by poison arrows that slowly petrify him.
Seto and his mate had agreed to keep it a secret, and thus his son Nanaki never learns from his mother what happened to his father. The cave is sealed, fulfilling Seto's last wish that everyone forgets about it.
Nanaki's mother died shortly afterwards, and he is raised by Bugenhagen. Oblivious of what happened, he believed his father was a coward who ran away instead of protecting his mate.
When Nanaki later returns to Cosmo Canyon as part of Cloud's party, Bugenhagen decides to show him the truth about his father. He leads them through the Gi Cave, which is haunted by the ghosts of the warriors who were killed in the battle. At the cave's rear entrance, Nanaki sees the petrified body of his father. Shame now replaced with pride, Nanaki vows to become a great warrior like him and fight with AVALANCHE in their struggle against Sephiroth. Nanaki's conversation with Bugenhagen is interrupted by tears shed by Seto's petrified remains.