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And I don't have to pay the mana cost

That is a staggeringly powerful effect, and if you can pair it (or another of the most powerful upkeep effects ever) with something that gives you additional upkeeps, such as Obeka, Splitter of Seconds (one of the best Grixis commanders for Brawl) or Shadow of the Second Sun, youll get even more value

This is not an endorsement of certain decks over others as far as their competitive viability goes

Having cheap artifacts around to tap makes this not really affect you and so very good at buying you the time you need

After the tournament begins, Kaiba starts to watch the contestants to see do they make anything suspicious

Notably, all "Mayakashi" Main Deck monsters prevents the player from Special Summoning non-"Mayakashi" monsters from the Extra Deck, either as a Continuous Effect, or as a Condition attached to their effect activations