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Augment can (and usually does) change the name, card types, subtypes, rules text, and power/toughness. The combined creature will have (at least) two artists and may now have multiple colors. Anything covered up in the augment process doesn't count, so ignore things to the left of the "metal bar" in the art of host creatures.
Creatures with augment don't have a mana cost and can't be cast.
You can't put more than one augment card on a single host creature. Once a host creature is augmented, the host part gets covered up and it's no longer a host creature.
Augment is an activated ability that you activate from your hand. To do so, reveal the card, choose a target host creature, and pay the augment cost. As this ability resolves, if the card with augment is still in your hand, put it onto the battlefield combined with the host creature.
The creature card with augment isn't put onto the battlefield until the ability resolves. This means if the host is destroyed, the creature with augment stays in your hand. You can't choose a different host, but you can activate augment again if there's another host available.
You can't activate augment unless there is a host creature on the battlefield. It doesn't need to be yours. Note though that if you augment another player's host creature, they control the combined creature.
Augment can't target creatures that aren't host creatures.
Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).
A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.
A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.
