The foul hydra is the alternate upgrade of the hydra, the fifth-level creature of the Dungeon, in Heroes of Might and Magic V: Tribes of the East. The foul hydra is very similar to the other upgrade, the deep hydra; a Large Creature with Enraged, Six-headed attack and No Enemy Retaliation. The difference between them is that the foul hydra has slightly worse stats, and instead of regeneration, it has Acid Blood: Every time the foul hydra is attacked, it does 25% of its normal damage to the attacker and the enemies around.