Spike is a very mature dragon. He can watch after himself when Twilight isn't home. He knew the rules. 'Don't accept a mysterious card game called 'Dimensional Royale' isn't one of those rules. Certainly this meager game isn't dangerous. It's a pack of playing cards and a game board-what harm could possibly come from that?

A lot of harm as a matter of fact.

A crossover between MLP, Gravity Falls, How to Train Your Dragon, and whatever other fandom I decide to add. Rated 'Teen' for minimal and cartoony violence.