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