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Dredge can replace any card draw, not just the one during your draw step. One card draw can't be replaced by multiple dredge abilities.

If an effect puts a card into your hand without specifically using the word "draw," you're not drawing a card. Dredge can't replace this event.

If multiple lands enter the battlefield under your control simultaneously, all of those lands are counted. For example, if you sacrifice seven lands while resolving Scapeshift and search your library for seven lands with different names, you'll create seven Zombie tokens.

If you control multiple lands with the same name, only one of those lands will count toward the seven or more required to create a Zombie. For example, if you control four lands named Forest, two named Plains, and one named Swamp, you control three lands with different names.

If you're drawing multiple cards, each draw is performed one at a time. For example, if you're instructed to draw two cards and you replace the first draw with a dredge ability, another card with a dredge ability (including one that was put into your graveyard by the first dredge ability) may be used to replace the second draw.

Once you've announced that you're applying a card's dredge ability to replace a draw, players can't take any action until you've put that card into your hand and put the top cards of your library into your graveyard.

You can't attempt to use a dredge ability if you don't have enough cards in your library.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

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.

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.).