Adventure

Adventure is a spell type, a subtype seen exclusively on instants and sorceries attached to creature cards in Throne of Eldraine.

Description
Casting the Adventure half of a card represents sending the creature on an adventure before putting it onto the battlefield. When the adventure resolves, the card is exiled. When exiled as part of the resolution, the creature may be cast from exile.

Adventure spells only exist on the stack. In all other zones, the characteristics of an adventure are ignored; only the characteristics of the creature card apply.

In theory, the rules would allow Adventures to appear on all non-creature spells.

Rules
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Rulings

 * An adventurer card is a creature card in every zone except the stack, as well as while on the stack if not cast as an Adventure. Ignore its alternative characteristics in those cases. For example, while it's in your graveyard, Giant Killer is a white creature card whose converted mana cost is 1. It can't be the target of the triggered ability of Mystic Sanctuary.
 * When casting a spell as an Adventure, use the alternative characteristics and ignore all of the card's normal characteristics. The spell's color, mana cost, converted mana cost, and so on are determined by only those alternative characteristics. If the spell leaves the stack, it immediately resumes using its normal characteristics.
 * If you cast an adventurer card as an Adventure, use only its alternative characteristics to determine whether it's legal to cast that spell. For example, if Giant Killer is exiled with the last ability of Vivien, Champion of the Wilds, you can't cast it as Chop Down.
 * If a spell is cast as an Adventure, its controller exiles it instead of putting it into its owner's graveyard as it resolves. For as long as it remains exiled, that player may cast it as a creature spell. If an Adventure spell leaves the stack in any way other than resolving (most likely by being countered or by failing to resolve because its targets have all become illegal), that card won't be exiled and the spell's controller won't be able to cast it as a creature later.
 * If an adventurer card ends up in exile for any other reason than by exiling itself while resolving, it won't give you permission to cast it as a creature spell.
 * You must still follow any timing restrictions and permissions for the creature spell you cast from exile. Normally, you'll be able to cast it only during your main phase while the stack is empty.
 * If an effect copies an Adventure spell, that copy is exiled as it resolves. It ceases to exist as a state-based action; it's not possible to cast the copy as a creature.
 * An effect may refer to a card, spell, or permanent that "has an Adventure." This refers to a card, spell, or permanent that has an adventurer card's set of alternative characteristics, even if they're not being used and even if that card was never cast as an Adventure.
 * If an effect refers to a creature card, creature spell, or creature that has an Adventure, it won't find an instant or sorcery spell on the stack that's been cast as an Adventure.
 * If an object becomes a copy of an object that has an Adventure, the copy also has an Adventure. If it changes zones, it will either cease to exist (if it's a token) or cease to be a copy (if it's a nontoken permanent), and so you won't be able to cast it as an Adventure.
 * If an effect instructs you to choose a card name, you may choose the alternative Adventure name. Consider only the alternative characteristics to determine whether that is an appropriate name to choose.
 * Casting a card as an Adventure isn't casting it for an alternative cost. Effects that allow you to cast a spell for an alternative cost or without paying its mana cost may allow you to apply those to the Adventure.