Activate Tawnos' second ability by paying ({1} magic symbol) ({W} magic symbol) ({U} magic symbol) ({B} magic symbol) , tapping it, sacrificing two artifact tokens and exiling Treasure Vault from your graveyard, causing you to create a token of Treasure Vault.
When the Treasure Vault token enters the battlefield, Retreat to Coralhelm triggers, untapping Tawnos.
Activate the Treasure Vault token's first ability by tapping it, adding ({C} magic symbol) .
Activate Tawnos' first ability by paying ({2} magic symbol) and tapping it, creating a copy of the Treasure Vault token and causing you to mill two cards.