oath of the gatewatch booster box but it also has similarities with hedrons Oath Of The Gatewatch Booster

If you cast this Alchemy instant in response targeting the Vesperlark, it could target itself with its ability to cause an infinite loop where it died and came back

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C Common The Fang of Critias X SPELL (This card is also always treated as "Legendary Dragon Critias".) Send 1 Trap from your hand or field to the GY, that is listed on a Fusion Monster that can only be Special Summoned with "The Fang of Critias" (if that card is Set, reveal it), then Special Summon that Fusion Monster from your Extra Deck

Which is why Rhystic Study is the number one MTG best blue cards in commander format