This episode has the first film appearance of Cain, the host of the DC horror comic book series, "The House of Mystery."
The origins of The House of Mystery are unknown, but it appears to usually exist simultaneously in both the realm known as The Dreaming and on Earth, next to a graveyard (the House of Secrets stands opposite it). The House appears to be sentient, and to act as a repository for stories, normally quite horrific ones.
John Constantine won possession of The House of Mystery in a card game, but after the Justice League Dark (2017) faced Blight, John lost ownership to Zatanna.
For countless years, Cain, the first murderer, has acted as caretaker of The House of Mystery, as his brother Abel does The House of Secrets.
After the ten-year-old versions of the Justice Leaguers visit a house trick-or-treating, they compare what they received. The first three speak of receiving nice pieces of candy, while Kid Constantine complains, "I got a rock...candy stick." This is a twist on the bit in It's the Great Pumpkin, Charlie Brown (1966) when the Peanuts kids go trick-or-treating and Charlie Brown only gets rocks in his bag.