Rule 601.3e

If a rule or effect states that only an alternative set of characteristics or a subset of characteristics are considered to determine if a card or copy of a card is legal to cast, those alternative characteristics replace the object’s characteristics prior to determining whether the player may begin to cast it. Example: Garruk’s Horde says, in part, “You may cast the top card of your library if it’s a creature card.” If you control Garruk’s Horde and the top card of your library is a noncreature card with morph, you may cast it using its morph ability. Example: Melek, Izzet Paragon says, in part, “You may cast the top card of your library if it’s an instant or sorcery card.” If you control Melek, Izzet Paragon and the top card of your library is Giant Killer, an adventurer creature card whose Adventure is an instant named Chop Down, you may cast Chop Down but not Giant Killer. If instead you control Garruk’s Horde and the top card of your library is Giant Killer, you may cast Giant Killer but not Chop Down.

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