Spell type

A spell type is any subtype used for the further classification of instant and sorcery cards.

Other than creature types spell types don't have to appear on all sorceries and instants. Instead they are only introduced for mechanical purposes.

Rules
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Adventure
Adventure only appears on instant or sorcery spells attached to creature cards. The creature is said to have "gone on an adventure" if the spell was cast first, though all references to this are to disregard this requirement.

Arcane
Arcane spells are the main target of the Splice mechanic, and form half of the spiritcraft mechanic.

Lesson
When learning you may reveal a Lesson card from outside the game and put it into your hand.

Trap
Trap is only used on instants. They have an alternative cost should specific events occur. Usually, traps have abilities that undo or punish whatever has made their alternative cost possible.