The Great Downing is an event in Ga'Hoolian history. In the chronology of the books, it took place during The Journey . This is when the Guardians went out and 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, however, 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. As you will read under the history section, St. Aggie's raided the Pure Ones and took the owlets. However, the Pure Ones persued the St. Aggie's patrols and a small skirmish took place. The owlets were dropped and both groups were forced to leave without them.
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 took chase 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 were forced to abandon their stronghold.
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, however, 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 an area where it was impossible to navigate, a Devil's Triangle. Soren then led a mission to free him.