Fateweaver's Reversal

(You may cast a legendary instant only if you control
a legendary creature or planeswalker.)
If you would lose the game this turn, you win
the game instead. You lose the game at the
beginning of the next turn's upkeep.
Pay half your life, discard half the cards in your
hand, sacrifice half the permanents you control: Counter Fateweaver's Reversal. Any player may activate this ability but only if Fateweaver's
Reversal is on the stack. (Round up each time.)