Sea Witches are a magical race of human-octopus hybrids similar to how mermaids are human-fish hybrids, in that they have the upper half of a human (up to around the space where the genitalia are, thus requiring similar coverage), but with the legs replaced by 8 tentacles like an octopus. The Sea Witches were originally humans, spirited away by a Great Old One called "The Great Tentacled One" to his world Ronoran, changed to their new looks, and made to serve him. Since they are all-female magicians of the sea, they naturally called themselves the "Sea Witches".
While not much is known about the Cult of the Sea Witches and their society or habits, what the last surviving member Allura tells Eric(a) is that the Sea Witches are a matriarchal society (by virtue of them being all-female) and they regularly birth new members much like most other species do. One would assume they like taking human or human-esque lovers to do so, but the truth is actually a fair bit worse, in that the Sea Witches actually put their fertilised eggs into other species' females and make THEM birth new Sea Witches. Due to this, they are hated in many places as an infesting and corrupting influence.
The Sea Witches saw their last days a short while ago when the Magicos attacked Ronoran. Marianne's Magical Girls deployed immediately to provide aid in the battle, but between the endless hordes of monsters and the Mermaids (the dominant species of Ronoran who hate magic-using land dwellers and particularly so the females) attacking their own supposed allies, Marianne eventually called it off and the world was lost.
The Sea Witches however did not just stand by during that time. They instead called upon their God that was prophesised to make the stars themselves tremble when he awakens. Awaken him they did, and make the stars tremble he did... until a Magico Leviathan came along and casually tore him apart and ate the remains. The Sea Witches, now completely broken in spirit after seeing their God get killed so easily (even though he did cause a fair bit of damage to the invaders), for the most part threw themselves into the battle and died. Since then only Allura remained and later somehow found herself in the Pocket Dimension.
What the future holds for this species, nobody can say, as Allura herself has given up on her heritage (being the sole survivor) and without their God she thinks they are nothing and the history books will just forget them.
The following characters are Sea Witches: