Herald of Eternal Dawn
White has printed loss-prevention effects before, but almost always tied to a fragile permanent or a condition the opponent can dismantle: Platinum Angel dies to any answer and the lock evaporates, Angel's Grace lasts only until your next turn, Lich's Mastery hangs the whole game on a single creature. This one folds the same two clauses into a static ability that holds while the body stands, which changes the question from "how do I set up the lock" to "how do I keep a 6/6 flier alive." The flash is what makes the piece dangerous rather than durdly. A seven-mana wall against defeat that could only arrive during your own turn would be a build-around; instant speed lets it drop in mid-combat, in response to the burn spell or the lethal alpha strike that was supposed to close the game, and simply cancel the loss it would have caused. The clause cuts both directions cleanly: you can't lose while it survives, and opponents can't win, so mill and poison and alternate-win conditions all stall against the same statement. It does not stop your life total from falling, does not stop damage, does not stop the board from being overrun; it stops the game from ending on those things. Kill the Angel and the ledger comes due all at once, which is the tension that keeps a hard lock from being an auto-include. Until then, the game does not end.
