Take Thel'zan's Phylactery to Legion Commander Yorik inside the Wintergarde Mausoleum in Dragonblight.
Thanks to your heroics, Lord Fordragon has given the command to launch a full assault against Thel'zan. 7th Legion have made their way into the summoning chamber of the mausoleum and located what appears to be a gateway to hell. The only piece that's missing now is you.
Take the phylactery and report to Legion Commander Yorik inside the Wintergarde Mausoleum.
There is a tunnel that the Scourge dug out just outside the keep that you can take to get to Yorik quickly. Go now, [player]! Lead us to victory!