Slay Arix'Amal. Take the Burning Legion Gate Key. Use the Burning Legion Gate Key on the Rune of Spite.
1x Close Burning Legion Gate
You are an honor to us, [player]. But now we must strike before the Legion recovers! To the north of Thrallmar lies Invasion Point: Annihilator. It houses a warp-portal through which the Legion draws its reinforcements.
You must overload that portal - and blast it back to the abyss it came from. The portal's activation key will be held by the Point's commander - Warbringer Arix'Amal. Slay him. Take the key. Use it on the portal and do what must be done. May the warchief's blessings be with you.