Madness

Madness is a keyword ability on spells that allows a player to cast that spell for an alternate cost if the card is discarded. It is primarily found in black.

History
Madness was introduced in the Odyssey block in Torment, and has been featured in Time Spiral block  and Shadows over Innistrad. In the latter set, it is tied to the Vampire tribe. Madness also appeared in Commander 2019 and Modern Horizons 2.

Torment's madness cards inspired a deck known as "Wild Mongrel", or "UG Madness", which, using the signature card Wild Mongrel, enabled instant-speed discard, allowing cards with Madness to be played at instant-speed. Wild Mongrel also enabled other abilities such as flashback and the seven-card Incarnations (Anger, Brawn, Filth, Genesis, Glory, Valor, and Wonder).

Each reprinting of the mechanic has brought minor updates to the rules for the keyword. The Time Spiral update was part of a sweeping cleanup to the formatting of the Comprehensive Rules, and did not change the gameplay of the ability.

However, the Shadows over Innistrad update did subtly alter the process of discarding a card with madness. Previously, a player discarding a card with madness made two choices. First, that player could choose to discard the card into exile rather than the graveyard. Then, if the card was discarded into exile, that player could choose to cast the spell immediately for its madness cost, or to let the card continue to the graveyard as usual. The first part of this process is no longer optional: a card with madness is always discarded into exile, after which the player can choose whether to cast it, or let it continue to the graveyard. The mechanic also featured in the follow up set Eldritch Moon.

Rulings

 * When you discard a card with madness, you discard it but the card goes into the exile zone instead of the graveyard. Then, you may cast it, or you may put it into your graveyard. Casting a card with madness is just like casting an instant from your hand, except you pay the spell's madness cost instead of its mana cost. It goes on the stack like any other spell and it can be countered like any other spell.
 * When you cast a card with madness, it still counts as being discarded, but it doesn't actually get to your graveyard before you cast it. That means your opponent can't exile it "in response" to stop you from playing the spell. Abilities that trigger on a card being discarded, however, will still trigger.
 * When you cast a spell by paying its madness cost, its mana cost doesn't change. You just pay the madness cost instead.
 * Effects that cause you to pay more or less for a spell will cause you to pay that much more or less for its madness cost, too. That's because they affect the total cost of the spell, not its mana cost.
 * Madness works no matter why you're discarding the card. You could discard to pay a cost, because a spell or ability tells you to, or even because you have too many cards in your hand at the end of your turn. You can't discard a card with madness just because you want to, though.
 * If you choose not to cast a card with madness when the madness triggered ability resolves, it goes to your graveyard. You don't get another chance to cast it.

Notable cards with Madness

 * Basking Rootwalla</c>
 * Big Game Hunter</c>

Card that grants Madness

 * Falkenrath Gorger</c>

Card that interacts with Madness

 * Anje Falkenrath</c>

Non-related cards with Madness in name

 * Brink of Madness</c>
 * Descent into Madness</c>