Descent into Madness

• WWizards of the Coast2012-05-01
  Official Ruling
  First you choose the permanents and/or cards from your hand that will be exiled. Then each other player in turn order does the same. Then all the chosen permanents and cards are exiled simultaneously. Players who choose after you will know what permanents you’ll be exiling when they choose. They’ll know how many cards you’ll be exiling from your hand, but they won’t see those cards.
• WWizards of the Coast2012-05-01
  Official Ruling
  If Descent into Madness isn’t on the battlefield when its ability resolves, use the number of counters on it when it left the battlefield to determine how many permanents and/or cards from hands to exile.
• WWizards of the Coast2012-05-01
  Official Ruling
  If there are more counters on Descent into Madness than the total number of permanents you control plus the number of cards in your hand, you’ll exile all permanents you control (including Descent into Madness) and all cards from your hand.
• WWizards of the Coast2012-05-01
  Official Ruling
  For each despair counter on Descent into Madness, you’ll exile a permanent you control or exile a card from your hand, not both.

Rulings via Scryfall