Descent into Madness is a mythic Enchantment card from Avacyn Restored (AVR), collector number 97. Current market price: $0.35 (foil $1.77), TCGplayer market prices via Scryfall.
At the beginning of your upkeep, put a despair counter on this enchantment, then each player exiles X permanents they control and/or cards from their hand, where X is the number of despair counters on this enchantment.