The Seattle Underground is a real tour open to the public.
(at around 1h 21 mins) The patient report for Emily May is copied directly from the Wikipedia article on Craniopagus Parasiticus.
On October 24, 2019, James Wan clarified that the film is not based on his graphic novel Malignant Man, stating, "It's definitely not a superhero film. Malignant is an original thriller not based off any existing IP."
Nicolas Cage is a big fan of the movie calling it an inspiration and his favourite horror of the year.
Stephen King praised this movie on X [Twitter], writing "I watched Malignant on HBO and thought it was brilliant".