Madness

Madness is a keyword ability on spells that allows a player to play that spell for an alternate cost it is discarded. It was first introduced in the Odyssey Block in Torment.

Madness inspired a deck known as "Wild Mongrel", which, using the signature card Wild Mongrel, enabled instant-speed discard, which allowed cards with Madness to be played at instant-speed. Wild Mongrel also enabled other abilities such as Flashback and the seven-card cycle known as "The Incarnations" (Anger, Brawn, Filth, Genesis, Glory, Valor, and Wonder.)

From the Comprehensive Rules

• 502.24. Madness
  • 502.24a Madness is a keyword that represents two abilities. The first is a static ability that functions while the card with madness is in a player’s hand. The second is a triggered ability that functions when the first ability is applied. "Madness [cost]" means "If a player would discard this card, that player discards it, but may remove it from the game instead of putting it into his or her graveyard" and “When this card is removed from the game this way, until that player passes next, the player may play it any time he or she could play an instant by paying [cost] rather than paying its mana cost. When the player passes next, he or she puts this card into his or her graveyard."
  • 502.24b Playing a spell using its madness ability follows the rules for paying alternative costs in rules 409.1b and 409.1f–h.

For your reference [CR 409.1b,f-h]

• 409.1b If the spell or ability is modal (uses the phrase “Choose one —” or “[specified player] chooses one —”), the player announces the mode choice. If the player wishes to splice any cards onto the spell, he or she reveals those cards in his or her hand. If the spell or ability has a variable mana cost (indicated by {X}) or some other variable cost, the player announces the value of that variable at this time. If the spell or ability has alternative, additional, or other special costs (such as buyback, kicker, or convoke costs), the player announces his or her intentions to pay any or all of those costs (see rule 409.1f). You can’t apply two alternative methods of playing or two alternative costs to a single spell or ability. Previously made choices (such as choosing to play a spell with flashback from his or her graveyard or choosing to play a creature with morph face down) may restrict the player’s options when making these choices.
• 409.1f The player determines the total cost of the spell or ability. Usually this is just the mana cost (for spells) or activation cost (for abilities). Some cards list additional or alternative costs in their text, and some effects may increase or reduce the cost to pay. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost, activation cost, or alternative cost, plus all cost increases and minus all cost reductions. Once the total cost is determined, it becomes “locked in.” If effects would change the total cost after this time, they have no effect.
• 409.1g If the total cost includes a mana payment, the player then has a chance to play mana abilities (see rule 411, “Playing Mana Abilities”). Mana abilities must be played before costs are paid.
• 409.1h The player pays the total cost in any order. Partial payments are not allowed.
  • Example
    You play Death Bomb, which costs {3}{B} and has an additional cost of sacrificing a creature. You sacrifice Thunderscape Familiar, whose effect makes your black spells cost {1} less to play. Because a spell’s total cost is “locked in” before payments are actually made, you pay {2}{B}, not {3}{B}, even though you’re sacrificing the Familiar.