Were you looking for the Spirit Tower PvP objectives in the Bone Wastes?

Once infused with the restless spirits of the vengeful dead, spirit towers present a formidable defense. The wraiths of the tower attack with supernatural efficiency, and quickly add enemy forces to the ranks of dead. Multiple towers in an undead city will chill the heart of even the most courageous foe.^{WC3man 43} It is an upgraded form of the Ziggurat.