Polukranos, also known as World Eater, is a gigantic legendary Hydra from Theros.
First resting in a celestial lair, the monster fell to Theros because of the battle between Purphoros and Heliod. It was sealed under the Nessian Forest by Heliod and Nylea. When the planeswalker Elspeth Tirel came back to Theros, Heliod made her his champion, and sent her to defeat the Hydra, which had awakened and was ravaging the lands. The monster fell, the Sun's champion defeating it with Godsend, a swordspear crafted by Heliod using the same weapon which had made Polukranos fall from his former lair.
After its death Polukranos was locked away in The Underworld. During the war of the gods, the undead monster managed to break free.