Adventure is a spell type, a subtype seen exclusively on instants and sorceries attached to creature cards in Throne of Eldraine.
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.
From the glossary of the Comprehensive Rules (January 24, 2020—Theros Beyond Death)
- Adventurer Card
- Cards with a two-part card frame (one part of which is inset on the left) on a single card. See rule 715, “Adventurer Cards.”
From the Comprehensive Rules (January 24, 2020—Theros Beyond Death)
- 715. Adventurer Cards
- 715.1. Adventurer cards have a two-part card frame, with a smaller frame inset within their text box.
- 715.2. The text that appears in the inset frame on the left defines alternative characteristics that the object may have while it’s a spell. The card’s normal characteristics appear as usual, although with a smaller text box on the right.
- 715.2a If an effect refers to a card, spell, or permanent that “has an Adventure,” it refers to an object for which these alternative characteristics exist, even if the object currently doesn’t use them.
- 715.2b The existence and values of these alternative characteristics is part of the object’s copiable values.
- 715.2c Although adventurer cards are printed with multiple sets of characteristics, each adventurer card is only one card. For example, a player who has drawn or discarded an adventurer card has drawn or discarded one card, not two.
- 715.3. As a player casts an adventurer card, the player chooses whether they cast the card normally or as an Adventure.
- 715.3a When casting an adventurer card as an Adventure, only the alternative characteristics are evaluated to see if it can be cast.
- 715.3b While on the stack as an Adventure, the spell has only its alternative characteristics.
- 715.3c If an Adventure spell is copied, the copy is also an Adventure. It has the alternative characteristics of the spell and not the normal characteristics of the card that represents the Adventure spell. Any rule or effect that refers to a spell cast as an Adventure refers to the copy as well.
- 715.3d Instead of putting a spell that was cast as an Adventure into its owner’s graveyard as it resolves, its controller exiles it. For as long as that card remains exiled, that player may cast it. It can’t be cast as an Adventure this way, although other effects that allow a player to cast it may allow a player to cast it as an Adventure.
- 715.4. In every zone except the stack, and while on the stack not as an Adventure, an adventurer card has only its normal characteristics.
- 715.5. If an effect instructs a player to choose a card name and the player wants to choose an adventurer card’s alternative name, the player may do so.
- 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.