Fleet Swallower as an attacker, triggering its ability, causing a player to mill half their library, rounded up.
Activate
Riverchurn Monument's exhaust ability by paying ({2} magic symbol) ({U} magic symbol) ({U} magic symbol) and tapping it, causing each player to mill cards equal to the number of cards in their graveyard, thus causing the player targeted by
Fleet Swallower's ability to mill the remainder of their library.