Bring 5 Resonating Skulls and the Mystic Crystal to Carlin Redpath in Eastern Plaguelands.
Chromie said the relics you gathered are reagents to a powerful spell, and one more is required: skulls from the scourge who took part in the battle of Darrowshire.
Take this crystal and hunt scourge champions for their skulls. The crystal is enchanted; if a champion took part in the battle, then when you pass the crystal over its skull it will resonate. Bring me those resonating skulls.
Go, [player]. You'll find scourge champions in the Noxious Glade to the north and the Fungal Vale to the west.