Storm

Storm is a keyword ability that creates a copy of the spell for each spell cast before it in the current turn.

History
The first cards with Storm were printed in Scourge, with more being printed in Time Spiral, one in Commander (Flusterstorm) and one in Unstable (Crow Storm). Thousand-Year Storm from Guilds of Ravnica is an enchantment that gives its controller's instants and sorceries a modified version of storm.

Storm also appeared in Modern Horizons. In the Mystery Booster set it appeared on a test card. This was the first time it was used on an Enchantment. Three cards with Storm appear in the Mystical Archive, and in Strixhaven: School of Mages, the card Show of Confidence has a one to one function to cards with Storm.

Aeve, Progenitor Ooze from Modern Horizons 2 is the first black-border permanent with storm and the first creature. All copies become token copies of the creature.

Rulings

 * The storm copies are put directly onto the stack—they aren't cast. That means the copies don't generate storm copies themselves, and they aren't counted by other storm spells cast later during the turn.
 * For the same reason, an effect like Twincast or Beamsplitter Mage, which can copy a spell with storm, will only create one new spell. The copy's storm ability will not trigger because the spell was not cast.
 * Each storm spell with a target allows you to change the target for each copy of that spell. You make that choice for each copy individually.
 * When counting spells cast in a turn, you do count spells that were cast face down, spells cast from zones other than a hand, and spells that were countered.
 * A copy of a spell can be countered, just like any other spell, but each copy has to be countered individually. Countering a storm spell won't counter the copies of it.
 * Exiling a card using suspend doesn't count as casting a spell; you only cast a suspended card when you remove the last time counter from it and that ability resolves.

Notable cards with storm

 * Tendrils of Agony — Used in Vintage and Legacy storm decks as the main win condition
 * Empty the Warrens</c> — Used in Vintage, Legacy, and Modern storm decks as the main/alternate win condition and banned in Pauper
 * Brain Freeze</c> — Used in Legacy storm decks as the main win condition when used with High Tide</c>
 * Mind's Desire</c> — Used in Vintage storm decks with Tendrils of Agony as the win condition
 * Grapeshot</c> — Used in Modern storm decks as the main win condition and banned in Pauper
 * Dragonstorm</c> — Used in Ravnica/Coldsnap/Time Spiral Standard storm decks
 * Flusterstorm</c> — A counterspell with storm used in Legacy decks
 * Temporal Fissure</c> - Previous used in Pauper storm decks as the main win condition before its banning

Storm scale
Mark Rosewater has declared that he doesn't ever see storm coming back in a Standard-legal set. On Blogatog, he uses the Storm Scale—a scale of 1 through 10 where 10 is storm—when he answers questions about the likelihood of the return of a mechanic in a Standard-legal set. 1 = will definitely see again; 10 = never say never, but this is pretty close to never. The Storm Scale does not affect the reprinting of cards in supplemental sets such as Modern Masters and the Magic Online-only set Vintage Masters. It also didn't prevent the creation of Crow Storm</c> in the silver-bordered set Unstable or of Krosan Adaptation</c> as a Mystery Booster test card.