Shapeshifter's Marrow

The most blue-aligned way to imagine theft: not a clone you choose, but one your opponent's own library hands you when it slips. The enchantment sits inert until an opponent flips a creature off the top, at which point that card goes to the graveyard and this permanent inherits its identity, surrendering the trigger in the trade. It is denial wearing a transform costume. Every upkeep is a check the opponent runs for you, and when it hits, a creature is stripped from their library and reappears under your control sized to whatever they were drawing into. The design tension is entirely in who controls the timing: you do not pick the moment or the target, so the card reads as patient interference rather than reactive value. That passivity is the cost of acquiring a creature without paying its mana or its color, and of disrupting the opponent's draws while you wait. It works by punishing a known library state rather than answering the board in front of you, and it poses the question blue rarely asks out loud: how large can an enchantment become if it simply waits long enough for someone else to make the decision for it.