Wight of the Reliquary does not have summoning sickness.
You control at least one additional creature.
Steps
Activate Wight of the Reliquary by tapping it and sacrificing Bloodghast, causing you to search your library for a land card and put it onto the battlefield tapped.
When the land enters the battlefield, Bloodghast and Retreat to Coralhelm trigger.
Resolve the Bloodghast trigger, returning it from your graveyard to the battlefield.
Resolve the Retreat to Coralhelm trigger, untapping Wight of the Reliquary.
Repeat.
Results
Near-infinite landfall triggers.
Put all land cards from your library onto the battlefield tapped.