You have a basic Forest, Mountain, or Swamp in library.
Steps
Play
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Riveteers Overlook.
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Riveteers Overlook enters the battlefield, triggering itself and
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Retreat to Coralhelm.
Resolve the
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Riveteers Overlook trigger, causing you to sacrifice it, then search your library for a basic Mountain, Swamp or Forest card, put it onto the battlefield tapped, and gain 1 life.
The basic land enters the battlefield, triggering
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Retreat to Coralhelm, tapping
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Scaretiller.
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Scaretiller triggers, returning
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Riveteers Overlook from your graveyard to the battlefield tapped.
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Riveteers Overlook enters the battlefield, triggering itself and
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Retreat to Coralhelm.
Resolve the
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Retreat to Coralhelm trigger, untapping
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Scaretiller.
Repeat from step 3.
Results
Near-infinite landfall triggers.
Near-infinite lifegain.
Near-infinite lifegain triggers.
Put all basic Swamps, Forests, and Mountains from your library onto the battlefield tapped.