yugioh stardust accelerator deck / Synchron from 𝓥𝓮𝓬𝓽𝓸𝓻 Yu-Gi-Oh! 5D's Stardust Accelerator Part

Basically, they sit outside the game (but not in a sideboard slot), get put into the Command Zone, and from there they generate effects

When resolving an opponent's card effect that targets an "Archfiend" monster you control, roll a six-sided die and negate that effect if you roll a 1, 3, or 6, and if you do, destroy that card

Star Seraph Scale X LIGHT Level 4 [ FairyEffect ] ATK 1500 DEF 900 When this card is Special Summoned: You can Special Summon 1 "Star Seraph" monster from your hand, then you can place 1 LIGHT monster from your Graveyard on top of your Deck

Its a touch slower than something like Atarka Red but plays much better cards and has a much better late game

Etali, Primal Storm becomes this Gruul abomination thanks to the power of Phyrexia

For Sinnoh however, it was an anime original location and no where near where the Elite Four and Champion were fought in the games