|“||There are innumerable tales of multi-headed monsters, all springing from the actual entity of whose real existence a few have known through the ages. This creature did not originate on earth, but in the gulfs Outside. It was... a vampiric entity, living not on the blood of its victims but on their heads — their brains... Through the eons this being has ravened in the abyss beyond our dimension, sending out its call to claim victims where it could. For this entity, by absorbing the heads and brains of intelligent creatures both of this world and of other planets, emerges with its powers and vitality greatly augmented.||„|
|~ The narrator of Hydra about the Hydra.|
The Hydra is an antagonist of the Cthulhu Mythos, a literary universe of shared fiction started by the late H. P. Lovecraft. It is one of the Outer Gods which resides in an alternate dimension and is the subject of the ritual described in the pamphlet On the Sending Out of the Soul, which allows people to perform astral projection, but at the cost of invoking the dread Hydra, who possess the person performing the ritual and forces them to do its bidding.
The Hydra is described as a vampiric entity of a kind, although it feeds "not on the blood of its victims but on their heads — their brains...", increasing its power by devouring their brains, causing their faces to appear within the Hydra. It appears an enormous sea of black ooze with a multitude of heads sprouting from it, some human, some alien, a new one appearing once another victim's head is devoured by the Hydra. The heads appear to be "sobbing and grimacing, as if in great agony".
The Hydra is worshipped by a human cult responsible for the creation of the On the Sending Out of the Soul astral projection pamphlet. Not much is known about the cult beyond the fact that they use the pamphlet to trick people into performing sacrifices for the Hydra.
In Henry Kuttner's short story Hydra, the Hydra's first and only appearance, it is revealed that its human cult often distributes a pamphlet named On the Sending Out of the Soul detailing how to perform astral projection. When followed, the formula always works as expected, harmlessly transporting the user in astral form to whatever destination is desired. However, unbeknownst to the user, the ritual also brings the subject into contact with the Hydra, which then merges with the individual's astral self, using it as a host. The traveller is then forced to bring somebody to the Hydra and decapitate them. The head is then absorbed by the Hydra and appears within it, the Hydra gaining more power from devouring the victim's brain. Afterwards, the astral traveller is returned safely to his or her original body, suffering no ill effects, except perhaps receiving a terrible shock from the grisly scene so witnessed.