According to this journal, Dar'Khan promised the Scourge two powerful elven artifacts: the stone of light and the stone of flame. These were imbued with the magical energies of the Sunwell and attuned to have warding powers against the Scourge.
The crystals were to be transported to where the Howling Ziggurat and the Bleeding Ziggurat now stand, in the middle of the Dead Scar. If they're still held there, we need to recover them. Their power will be greatly useful in our struggle against the enemy.