Bloodwrack Medusae

Not all Bloodwrack Medusae are goaded to war atop monolithic shrines. Some emerge willingly from their lairs and follow the scent of blood to war. They are unreliable allies who pay no heed to any battle plan, seeking only to share their own torment with their luckless victims.

Source

 * Warhammer Armies: Dark Elves (8th Edition) -- pg. 50.