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.
Illidan's attention is now on Kael'thas, thanks to you. The time has come to make our move.
I've had a vision of an artifact that will aid us in our final struggle - the phylactery of a lich known as Winterchill. Unfortunately its power was lost forever after his defeat in the Battle of Mount Hyjal.
There might be a way to retrieve this artifact, however. Legend speaks of a place in Azeroth guarded by bronze dragons... a place where past and present meet. It is there you will find what we seek.
Have you retrieved the phylactery yet, <name>?
My vision... it all becomes clear now. The powers this artifact has over a soul.... to contain it... to siphon it into itself... I will use those powers to reclaim what belongs to me! The time has arrived for us to take the fight to Illidan!