For example, even if we interpreted your library as part of the battlefield for this purpose, slipping one into the card sleeve of a card in your library during a search effectively marks your deck, and thats not allowed
Thankfully, with Wilds of Eldraines Enchanting Tales containing powerful white reprints like Land Tax and Smothering Tithe, Moonshaker Cavalry should have all the support it needs to have maximum impact and regularly close out games
You give your opponent a card, but you get a 3/2 to balance that out
For example, theres a Final Fantasy XV sidequest card called Catch a Fish that, upon completion, turns into a campsite where you cook up your catch
So if you can evolve into Shiftry on the first turn and you hit heads and shuffle your opponent's only Pokemon into their deck, then they lose automatically