In The Shadow Over Innsmouth H.P. Lovecraft introduced the deep ones; immortal ocean dwelling amphibians that traffic with humanity for dark, evil purposes. Over the years other authors expanded on Lovecraft’s mythology and the deep ones have shown up in numerous stories, games and even a film or two.
Grandmother Hydra here was inspired by the idea that deep ones don’t stop growing as they get older. I added the idea that old deep one sorcerers could mutate themselves by adding aspects of other sea creatures and then started sketching. Hydra seemed like she’d make a good adversary for King Roach.