This character first appeared in the Fairy Tale Rejects comic, but she's since been adapted into the comic's spiritual successor, a series of short stories called World of Looming Gaia. All the stories can be read for free here: https://archiveofourown.org/series/833844

Once a beautiful and vain elfenne, Morbus was tricked into kissing a melusine, thinking it would turn into a prince. But the melusine cursed her with a disease called Haggomah's Blight, which sapped her of all her beauty. Morbus has spent many decades since trying to find a cure for her affliction.

Morbus is a very talented alchemist. She can brew potions and medicines to cure almost any ailment, and can even cast pestilence spells on those who cross her. But her ghastly appearance has caused society to fear and shun her, and not many trust her enough to accept her medicines. She currently lives in a rotting tenement in the city of Tonsborg with her ten cats, brewing potions for her small but dedicated pool of customers. It is said--but never proven--that she also brews illicit drugs.