Trap

Trap is a spell type which is only used on instants. It was introduced in Zendikar, and also featured in Worldwake.

Description
Traps may be cast as for their regular mana cost, but also have an alternative cost should specific events occur. Usually, traps have abilities that undo whatever has made their alternative cost possible; for instance, a Trap that was enabled by two lands entering the battlefield under an opponent's control would destroy two lands.

New traps were designed for Zendikar Rising, but they didn't make to the final product.

Rules
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Trivia

 * The card Pitfall Trap was based on the artifact Pit Trap.
 * Trap was originally an artifact subtype in original Zendikar design. You played the card face down in exile informing the opponent(s) that you’ve laid a trap. Then if the opponent did a certain action you could flip it up and put it into play. Usually, it would negatively affect the opponent or their creatures in some way. Zendikar Rising attempted to bring back artifact Traps, simplifying the face-down exile aspect to an ability to put it into play from the hand without casting it. As mentioned above, it was later cut.