Does the game end immediately when one player reaches 0 life?
Player A loses.


Eighteen instances of Valakut's triggered ability will go on the stack.
Divine Deflection is cast and eventually resolves.
Trigger #18 resolves. It deals no damage and Divine Deflection deals three damage to Player A.
Trigger #17 resolves. It deals no damage and Divine Deflection deals three damage to Player A.
Player A loses the game for having non-positive life.
Player B wins the game for being the only player left.