Use the Sanctified Crystal against a wounded Uncontrolled Voidwalker and return to Elsaana at the Temple of Telhamat.
The blood elves epitomize the irresponsible use of magic, [class]. They do not control magical energy. Rather, they are slaves to it and will suffer nothing to stand between them and their addiction. They consume the energies of Outland's voidwalkers to power their demonic magics.
I've prepared a crystal charged with a spell that will allow us to neutralize a voidwalker and sever its link to this world when destroyed. I've mostly seen the voidwalkers in the Warp Fields to the southeast of Hellfire Citadel.