This episode's title is a reference to H.P. Lovecraft. Lovecraft's fictional town of Arkham, Massachusetts, setting for many of his stories, inspired the name of Arkham Asylum in previous Batman comics and movies.
The red head girl named Ivy is the comic book eco-terrorist known as Pamela Isley aka Poison Ivy who made her comic debut in Batman #181 (June 1966). She was a botanist who a failed experiment caused her body chemistry to become plant-like. She can give off poisonous gases, control plants, and has toxins that allow to control men.
Lesley-Ann Brandt who plays Larissa Diaz one of the assassins would go on to play Mazikeen on the show Lucifer, which is another DC comics TV show under their Vertigo line.
At 14:42 Harvey Dent mispronounces "mea culpa". He says MEEya; it should be "MAYya".