Shrine of Limitless Power

Text:

At the beginning of your upkeep or whenever you cast a black spell, put a charge counter on Shrine of Limitless Power.
, , Sacrifice Shrine of Limitless Power: Target player discards a card for each charge counter on Shrine of Limitless Power.

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 the last ability in response. The spell is on the stack by then.