The Great Mending happened in the 5th book, Spirit Wolf. It is said that a great glacier called H'rathghar scraped the eastern side of the Beyond, causing earthquakes everywhere and also resulting the Broken Talon point to be crushed. The resulting earthquakes killed most of the dire wolves and destroyed almost everything on the Beyond, including the Ring of Sacred Volcanoes along with the Ember of Hoole.
With the destruction of the ember, all the malcadhs (dead and alive) of the Beyond have their deformities fixed, like Faolan has his splayed paw fix and the Whistler now has a clear and fluent voice, though he does have difficulty in howling with his new throat. All malcadhs are whole again, except for Edme who is not born a malcadh, but made a malcadh by Dunbar MacHeath when she was a pup.