Activate Avenging Druid by tapping it, dealing 1 damage to an opponent.
Avenging Druid triggers, causing you to reveal a land card from your library and put it onto the battlefield, and put all other cards revealed this way into your graveyard.
When the land enters the battlefield, Retreat to Coralhelm triggers, untapping Avenging Druid.
Repeat until no lands remain in your library and/or your library is empty.
Results
Near-infinite damage.
Infinite self-mill.
Near-infinite landfall triggers.
Put all land cards from your library onto the battlefield.