Duplicity

A hand-swapping enchantment that asks you to give up the security of holding cards in exchange for a parallel hand stashed in exile, and the design is really a study in deferred information. The top five cards leave your library unseen, and from then on each upkeep offers a clean exchange: bank your current hand and take the other one back. The clever part is the discomfort it builds in. You never quite know what you are trading for until you commit, and the end-step discard means the swap is not free; you are bleeding a card every turn unless the exchange refills you. The final clause is the structural anchor that keeps it honest: lose control of the enchantment and every card it is babysitting goes to the graveyard, so the whole stockpile is hostage to the enchantment's survival. That turns a card-advantage engine into a target, and an enchantment-removal spell into a two-for-one or worse against you. It sits in the long tradition of blue control cards that hoard and cycle resources rather than draw outright, but where most of those just dig, this one toggles between two hands and dares the opponent to break the toy before you cash it in.