Mythology Gag/Tabletop Games

Tabletop RPG

 * A literal mythology gag appears in Dungeons & Dragons -- Minotaurs are immune to the spell Maze, which traps its victims in a labyrinth.
 * Dungeons & Dragons 4th Edition 4e loves tossing in shout-outs to settings that are not part of this edition's gameline. Sigil is once again center-stage at higher tiers, the Spelljamming Helms can be used to navigate the Astral Sea, and it's made throw-away references to defunct settings like Greyhawk and even Dark Sun before it was confirmed as a campaign setting for this edition.
 * In Vampire: The Masquerade, the Camarilla was the ruling body of vampiric politics -- a worldwide conspiracy meant to keep the Kindred operating behind the scenes and hidden from mortals. In the reboot, Vampire: The Requiem, the Camarilla was a vampiric body politic that stretched throughout the Roman Empire... and fell when it did, emphasizing the game line's shift from global to local politics.
 * The origin of the Free Council order in Mage: The Awakening occurred when they refused the Seer of the Throne ministry known as the Union's invitation to unite and create a world-controlling Technocracy.
 * Werewolf: The Forsaken has a giant worm-like entity known as Zmai who may or may not have something to do with vampires.
 * The Return of the Scarlet Empress book for Exalted had the Ebon Dragon construct an enormous Black Spiral. In addition, the entire world of Autochthonia is based on Mage: The Awakening. These qualify as Mythology Gags because Exalted was originally supposed to be a prequel to the World of Darkness, which has more or less fallen off with subsequent editions.