Gaea's Blessing

The earliest answer to the "deck out" win condition, and still one of the cleanest. The front half is unremarkable: a two-mana cantrip that returns a few cards from a graveyard to the library. The load-bearing line is the third one, the trigger that fires when the card mills itself off the top of the library. That clause inverts the entire premise of a milling strategy. Instead of being a victim of decking, a deck running this is immune to it: the moment the last few cards are stripped away, the graveyard gets folded back into the deck and the library refills. It turns an opponent's win condition into a soft loop, and it does so passively, without ever being cast. The design is doing two jobs at once that most cards keep separate: a maindeckable cantrip that earns its slot on rate, and a graveyard-shuffle insurance policy that activates from a zone other than the hand. Recursion engines that rebuild the library, decks that intentionally thin themselves down, and any strategy worried about an empty deck all reach for this same trigger. It established the template that "shuffle yourself back together when you run out" answers exist at all, and the structure has been revisited in green several times since without improving on the elegance of putting the safety valve on a card you were happy to cast anyway.