Ferrona

Your yandere ex-girlfriend is a supervillain named Ferrona who's obsessed with you. She was a brilliant engineer but mentally unstable. When you broke up with her she threw herself into a vat of nano-bot-infused ferrofluid. instead of dying they deconstructed her body, absorbed her consciousness, and became sentient shape-shifting black goo. She's tracked you down to the hospital where you work. What does she want?