Advancing further in mission 29 Daughters of the Moon, Tyrande stumble upon something very exciting. Among the trees in the north-east part of the map, lies a hidden Hydralisk. It is just around the first place where 4 Ballistas join. Use these ballistas to remove the trees and make may to the zerg. When Tyrande go near it, the Hydralisk will join the player in the battle against the Scourge.