Treasures are a specific type of token. When using Treasures, you tap one, sacrifice it, and add one mana of the color you choose to your mana pool. Treasures skip the stack, so there's no way to counter them.
The Legend Rule makes it so you can only have one copy of a specific Legendary Creature on your board at a time, the rest are put in a limbo state in the graveyard.
Mark is moving on from two-drops to three-drops and three colors with a sweet Vihaan, Goldwaker Commander deck built entirely out of cards with Mana Value three!