The Great Downing is an event in Ga'Hoolian history. In the chronology of the books, it took place during The Journey, when the Guardians found dozens of owlets on the ground, all chanting about Tytos and their purity. It was a great mystery as to how these owlets got here and where they came from. All of them were a species of Tyto, specifically Barn, Sooty, Masked, and Grass Owls. One of these owlets was Soren's sister, Eglantine, who, like all the other owlets, was babbling about the purity of Tytos and how sad she was that they had forsaken her. It was later found out that all these owlets came from a newly discovered and highly dangerous group known as the Pure Ones.
Eventually, it is revealed that St. Aegolius Academy for Orphaned Owls discovered the location of the Pure Ones' castle stronghold and raided it, taking all of the brainwashed owlets that the Pure Ones had snatched. The Pure Ones pursued the St. Aggie's patrols and engaged the owls in battle, forcing them to drop the owlets. The Pure Ones won the battle but were unable to find the owlets, and both groups were forced to leave without them, with the Pure Ones abandoning their stronghold in case they were discovered.
Shortly after, a patrol from the Great Ga'Hoole Tree found the many dozen missing owlets and bore the fallen owlets to the Great Tree for them to heal. They were, still babbling about the purity of Tytos, the result of being stone stunned by the Pure Ones. Soren accidentally discovered that looking through isinglass, or mica, returned the owlets to their proper state, but erased the memories of their time being stone stunned.
Ezylryb suspected Pure One involvement and set out to discover more information, but was imprisoned in a Devil's Triangle. Soren later led a mission to free him.