Akahyou no Sanctuary

"If you expose our secret, I shall kill you." After an accident on a mountain, Takatou is helped by Ritsu, a woman with a red birthmark under her right eye. "If consumed, they become an elixir." Having learnt the legend of the Red Leopard, Takatou seems to become obsessed and continues to search for Ritsu. Thus begins the love romance of these two trapped in a legend.

(Source: Kodansha, translated)