High Priest Orglum wants you to journey into the Tomb of Lights and use the Draenei Tomb Relic near the Ancient Draenei Altar. Defend the Tomb Guardian and defeat the Vengeful Harbinger. When the ritual is complete, speak with the Draenei Ascendant at the Ancient Draenei Altar.
The Tomb of Lights has become corrupted by the Shadow Council and the draenei dead have become cursed spirits, angry and confused with a burning hatred for the living.
Before I left Shattrath I was given a draenei tomb relic. It can be used near the Ancient Draenei Altar deep within the Tomb of Lights to summon a tomb guardian. This creature will activate the altar and will begin to purge the tomb of corrupt spirits, freeing them to ascend. Protect the tomb guardian until the ritual is complete.