Amanar

In the murky deeps far below Lothern's Emerald Gate slumbers Amanar, ancient protector of the city. Only in hours of greatest need does the merwyrm rouse and rise to the surface to consume those who would bring about Lothern's ruin. The records kept in the Glittering Tower tell that Amanar has manifested but three times in recorded history. The first was during the Daemon invasion, where he swallowed a whole legion that laid siege to the city. The second came during the Sundering, where his broad back held Lothern high above the tidal waves that swamped Ulthuan. The third, and to date final, appearance of Amanar came during the Great Incursion. With a great sweep of his tail, he scattered much of the Dark Elf army; then, ignoring the pinpricks of their blades, he turned and bore the Black Ark Intolerable Delight beneath the waves, never to be seen again.

Source
1a: pg. 64
 * Warhammer Armies: High Elves (8th Edition)