The Four Hydras is the collective name of the four great hydras that rule over the four seas of the world. They are all the direct offspring of the Goddess of Water, Akasha herself. Each of them was a head of the Sea Goddess, and were removed to carry on her legacy after her banishment.
The Western Hydra
The Western Hydra's domain was the Western Sea, including the waters around The Westfold and even western Haven. The smallest and youngest of the four, it was slain by the combined might of 3 Elementals and a dragon.
The Eastern Hydra
With dominion over the Eastern Sea, the Eastern Hydra directly took vengeance over the death of it's younger sibling. It possessed Erykas for over a month, all in an attempt to destroy Taylor with her own two hands. It was defeated and forcefully exorcised from her by the Rage entity. It is still alive, but has not been seen since.
The Northern Hydra
Ruling over the arctic Northern Sea, the Northern Hydra has not been seen in the story so far. There are several Viking clans that worship it.
The Southern Hydra
Making its home in the Southern Seas, the largest body of water in the world, the Southern Hydra is the oldest and by far the largest and most powerful of the four. While not having been directly seen, it caused a maelstrom several miles across.