Mythomagic

Mythomagic is a collectible card/figurine game formerly played by Nico di Angelo and some Roman Legionaires. It involved trading cards and miniature figurines, and is focused on Greek mythology.

The rules appear to be very complicated, with similarity in rules to the rules of real-life card games like Magic, the Gathering and tabletop role-playing games like Dungeons & Dragons. When Nico met a mythological creature, he would often say something about their abilities within the game or whether or not he had that particular item.

The Titan's Curse
Nico stops Dionysus from attempting to kill Percy by telling him about his card and how Dionysus is considered the weakest god card by most players, but how he thinks his powers are "totally sweet". Also, in order to distract Nico, Artemis asks Grover to have Nico explain the game to him.

The Battle of the Labyrinth
He is seen burning the Mythomagic cards and calling it a childish game in a dream of Percy's. Near the end of the book, Percy taunts Nico by saying that Hades has 4000 attack points, but Nico corrects saying he has 5000, if the opponent attacks first.

The Son of Neptune
Percy, Hazel, and Frank see several First and Second Cohort members playing Mythomagic inside their base when they should be guarding their banner during the Roman War Games. Their version of the card game belived to have features the Roman aspects of the gods.

Hades Figurine
In Mythomagic, the Hades figurine was the only figurine that Nico didn't have. Bianca took one from The Junkyard of the Gods, causing the prototype Talos to awaken and costing Bianca di Angelo her life. Talos was a creation made by Hephaestus. Percy tried to convince Nico to take it by saying that it had 4000 attack power, but Nico corrected him by saying it had 5000, but only if the opponent attacks first.

In Camp Half-Blood (Real world)
Some of the Camp Half-Blood Literary camps across the country, most notably the one in Austin, Texas and Decatur, Georgia have created their own versions of Mythomagic based off of the one in the book. Campers (who each control one hero) team up against a single counselor (who controls all the monsters). The game is played on Hero click boards, a miniature Greek temple, or a pyramid set (for Camp Kane, but it can be used for Camp Half-Blood also). Every year they play a championship, in which four random players are chosen to fight a boss battle I.E.: A Titan, Cerberus, river demons, a tank, etc. They have only had one in which nobody has died. The winner (whoever strikes the final blow) of the championship gets their own figure based off of them. However, because the merchandising rights to the series were sold to Fox (along with the movie rights), no official version has been released to the general public.