Monster Battles is a collectible card game with various monsters, and fictional heroes and villains depicted on the cards. Players battle each other via attacking and defending with monsters or "casting" spells against each other. Monster Battles became one of Harold's main hobbies after joining the Card Games society during Orientation Week. Harold is later depicted battling the president of the Card Games society Vryn Benalia in Next Steps.
Monster Battles is highly influenced by the real world game Magic: The Gathering which both Ben and Ross are fans of in real life, and coincidentally is produced by Wizards of the Coast, the publisher of Dungeons and Dragons. The president of the Card Games society Vryn Benalia is a subtle nod to this as both Vryn and Benalia are locations in the fictional multiverse in which the lore of Magic: The Gathering take place.