Go to the Caverns of Time in Tanaris and gain access to the Battle of Mount Hyjal. Once inside, defeat Rage Winterchill and bring the Time-Phased Phylactery to Akama in Shadowmoon Valley.

Chain

The Secret Compromised → Ruse of the Ashtongue → An Artifact From the Past → The Hostage Soul → Entry Into the Black Temple → A Distraction for Akama / A Distraction for Akama → Seek Out the Ashtongue → Redemption of the Ashtongue → The Fall of the Betrayer