Dementia Bat

Text:

Flying
, Sacrifice Dementia Bat: Target player discards two cards.

Rulings:

• 6/1/2011 Unlike most abilities that force a player to discard cards, this ability may be activated whenever you could cast an instant, including during your opponent’s draw step after they have drawn a card.
• 6/1/2011 You can’t force an opponent to discard a card they cast by activating Dementia Bat’s ability in response. The spell is on the stack by then.