Evoke

Evoke is a keyword ability that allows a player to pay an alternative cost for a creature spell that possesses this ability; however, if the evoke cost is paid, the creature is sacrificed when it enters the battlefield.

Description
Evoke was introduced in Lorwyn and expanded upon in Morningtide. It began in design as a mechanic that would allow an instant or sorcery to become a creature if an extra cost was paid, but was changed to its current form due to rules limitations. All creatures in Lorwyn with evoke have a "enters the battlefield" ability. The creatures with evoke in Morningtide all have "leaves the battlefield" effects. Evoke was reused sparingly in Modern Horizons and Modern Horizons 2.

Evoke allows you to play the spell for its evoke cost rather than paying its mana cost, but if you do that, you have to sacrifice it when it comes into play. This enables the player to pay the cheaper cost to just get the creature's "enters the battlefield" (or "leaves the battlefield") ability.

Evoke is similar, but not identical, in concept to channel. Both are associated with powerful immortal creatures (elementals and spirits, respectively), but whereas channeling a spirit merely allows a planeswalker to tap into that spirit's power without actually summoning it, evoking an elemental actually brings the elemental into existence for a very brief time, whereupon its power is expended in a single moment.

In theory, the rules would allow Evoke to appear on other permanents than creatures.

Rulings

 * Casting a creature by paying its evoke cost will result in two enters-the-battlefield abilities: The sacrifice ability from evoke, and whatever other ability the creature has. The creature's controller chooses in what order to put them on the stack. Both abilities can be responded to as normal.
 * Evoke doesn't change the timing of when you can cast the creature that has it. If you could cast that creature spell only when you could cast a sorcery, the same is true for casting it with evoke.
 * If a creature spell cast with evoke changes controllers before it enters the battlefield, it will still be sacrificed when it enters the battlefield. Similarly, if a creature cast with evoke changes controllers after it enters the battlefield but before its sacrifice ability resolves, it will still be sacrificed.
 * When you cast a spell by paying its evoke cost, its mana cost doesn't change. You just pay the evoke cost instead.
 * Effects that cause you to pay more or less for a spell will cause you to pay that much more or less while casting it for its evoke cost, too. That's because they affect the total cost of the spell, not its mana cost.
 * Whether evoke's sacrifice ability triggers when the creature enters the battlefield depends on whether the spell's controller chose to pay the evoke cost, not whether they actually paid it (if it was reduced or otherwise altered by another ability, for example).
 * If you're casting a spell "without paying its mana cost," you can't use its evoke ability.