The Ancient Treasures are a special set of cards used by Tenzen Yanagi in Yu-Gi-Oh! 5D's. According to the anime, it is possible that they are illegal, but currently that is not a problem. This set focuses on monsters with 0 ATK and 0 DEF to do its bidding, and nearly all of those cards have a negative effect (paying Life Points, discarding, etc.). These cards work well with "Curse Reflection Doll" and "Triangle O" to deflect the negative effects, however.
The cards are based on ancient objects from various locales around the world. That theme would later be reused in the form of the "Chronomaly" archetype.