Kicker

Kicker is a keyword ability that allows the player to pay an optional cost when casting a spell to achieve an additional effect.

History
Most early tournament-legal cards with kicker were printed during Invasion block. In this block the five-colored Coalition Symbol, or at least part of it, is buried in the art of most cards with the kicker ability. Kicker was brought back in the Time Spiral and Zendikar blocks. It also appeared in Dominaria , Modern Horizons and Zendikar Rising.

Rules
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Multikicker
undefined Multikicker, introduced in Worldwake, is a variant on Kicker. Multikicker functions exactly the same as Kicker, except that the Multikicker cost can be paid multiple times and the effect generated from it is duplicated for each time the cost is paid. Multikicker made a reappearance in Modern Horizons 2.

Rulings

 * You kick a spell as you cast it. You declare whether you're going to pay a kicker cost at the same time you'd choose a spell's mode, and then you actually pay it at the same time you pay the spell's mana cost. Kicking a spell is always optional.
 * You can pay any particular kicker cost only once, unless the card says "multikicker" instead of kicker. You can't pay it multiple times to "pump up" the effect if it is not multikicker.
 * Some instant and sorcery spells have an additional effect if they were kicked. Other instant and sorcery spells have a different effect if they were kicked. Read these cards carefully: If they include the word "instead," the second effect replaces the first. If they don't include the word "instead," both the first and second effects occur.
 * Some permanents with kicker enter the battlefield with counters on them if they were kicked. Other permanents with kicker have "enters the battlefield" triggered abilities that check whether they were kicked. These look at whether they were kicked when cast as a spell. If not, the ability doesn't trigger at all. If such a permanent is put onto the battlefield as the result of a spell or ability, there's no opportunity to kick them.
 * If a permanent has a targeted "enters the battlefield" ability that triggers if it was kicked, the target isn't chosen until the permanent enters the battlefield and the ability triggers (as opposed to when that permanent was cast). That means that sometimes you may wind up targeting something you don't want to. For example, say you cast a kicked Heartstabber Mosquito, which has the ability "When Heartstabber Mosquito enters the battlefield, if it was kicked, destroy target creature." In response to the Heartstabber Mosquito spell, your opponent sacrifices their only creature. When the Mosquito enters the battlefield, its ability triggers, and you must choose a target for it. If no one else controls any creatures, you must target one of your own—possibly Heartstabber Mosquito itself.
 * Kicker costs don't change a spell's mana cost or converted mana cost.
 * If a kicked spell is copied, the copy is also kicked.
 * Older cards with kicker were printed with abilities that had the text "if you paid the kicker cost" or "if its kicker cost was paid." Cards that were printed with that text have received errata in the Oracle card reference so they now say "if it was kicked." The cards work the same way they always did; these abilities have their effects if the player decided to pay the kicker cost, not necessarily if the listed cost was actually paid.