Sing a song of Mother Hydra.
(Hydra is worshiped in various names in various places across the galaxy and the multiverse - it is entirely likely you might find Her cults among Lor refugees, turning to the power of great and terrible Change to try and restore all they have lost)
Hesiod, Theogony [corrected]
"And in a hollow cave she bore another monster, irresistible, in no wise like either to mortal men or to the undying gods, even the goddess fierce [Hydra] who is half a nymph with glancing eyes and fair cheeks, and half again a huge snake, great and awful, with speckled skin, eating raw flesh beneath the secret parts of the holy earth. And there she has a cave deep down under a hollow rock far from the deathless gods and mortal men. There, then, did the gods appoint her a glorious house to dwell in: and she keeps guard in Arima beneath the earth, grim [Hydra],  a nymph who dies not nor grows old all her days."
Mother Hydra, the mother of all monsters, isn't hostile to humanity. In a sense. Is the ocean hostile? Is a hungry shark? Is a tsunami? Is eminent domain hostile to landholding? She is Progress - not hate. And She is old - much older than her consort Father Dagon, ancient even when the Serpent-Men of Lemuria and their Deep One spawn ululated Her name and gyrated unwholesomely beneath the blood-red Moon.
She Was, She Is, She Will Be.
When Creation happened, fragments of other Things from the chaos Before survived - some have names most Unspeakable, others have names given to them by other beings in the uncountable eons that followed, all are in some ways aspects of that unspeakably primordial Chaos. If the Unspeakable One is chaos in its ultimate, destructive aspect, Mother Hydra is simply the power of chaos to change - of waves to reshape mountains into beaches, of a fire to burn away a dead forest to let a new one grow, of a plague to sweep away a civilization and let a new people thrive in its place.
HAIL HYDRA! ETERNAL HYDRA! CUT OFF ONE HEAD AND TWO MORE WRITHE TO TAKE ITS PLACE!
It's hard to summon Hydra - that's why Dagon is usually the one that shows up when the cultists have made the sacrifices and performed the unholy rituals. When She does deign to show herself, it's usually during a great crisis - as when Her champion the Deep One Aquaria Innsmouth summoned Her during the recent invasion of the Terminus. Hydra's summonings in this plane generally take the forms of wet, slithering limbs reaching out from the darkness to grab and crush - or to drag away to the dark places where beaked mouths await to feast. You can cut off those limbs, you can patch the weakened patches in the universe that let her come through, but you can't strike at the Mother herself.
Halogen: You know where She lives, and no one's ever tried to take Her out?
Eldrich: People have tried.
One becomes the Mother of All Monsters by mothering - by spawning and by making more. If Hydra is chaos she is also fecundity, the power to grow and grow, to spread and change, in all its infinite and terrible glory. (If Her scion Apep were to eat the Sun She would neither be angry or pleased - what She cares about is whether or not he's making lots of little snakes. After all, Heqet is Her scion too.) It is this more than anything else that makes her an antagonist of the Unspeakable One (and what made Her answer a summons when the world was threatened) - how can there be growth, how can there be change, if all is destroyed?
"The old has passed away; behold, the new has come."
When the stars are right and Dagon and Hydra rise, the Earth will be filled with the glory of Their coming and the waters of the ocean will cover the land. The first will be last and the last will be first, and the old order and all it represents will be swept away in the great day of Jubilee. (Hydra is one of the major reasons why there is a Pact - and specifically why many of the deities attuned to humanity signed off on it.)
The testimony of Zadok Allen [corrected]
"Mother Hydra an'Father Dagon what we all come from onct”
There is some confusion about traditional images of the Dark Mother and Dark Father. What some have seen as a picture of a singular entity is actually two - Dagon is the "scaly, rubbery-looking body [with] prodigious claws on hind and fore feet," of the tales, but it is Hydra who is the "octopus-like head with a mass of feelers, [with] long, narrow wings behind." The traditional statues show they sit on Their throne together. As They might. When the stars are right.
Edited by Avenger Assembled