You have a basic Forest, Plains or Mountain in library.
Steps
Play
Cabaretti Courtyard.
Cabaretti Courtyard enters the battlefield, triggering itself and
Retreat to Coralhelm.
Resolve the
Cabaretti Courtyard trigger, causing you to sacrifice it, then search your library for a basic Forest, Plains or Mountain 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
Cabaretti Courtyard from your graveyard to the battlefield tapped.
Cabaretti Courtyard 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 Forests, Mountains, and Plains from your library onto the battlefield tapped.