Kalithal, Torment Unbound

Flash(You may cast this spell at any time you could cast an instant)
Spells do not resolve while Kalithal is on the battlefield. Spells stay on the stack.
At the beginning of each players upkeep they may resolve a spell of their choice that is on the stack.
Whenever a player resolves a spell that player discards a card.
{x}{u}{b},Exile Kalithal: X spells don't resolve this turn.