Terisian Mindbreaker as an attacker, triggering its ability, causing an opponent 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 attacked by
Terisian Mindbreaker to mill the rest of their library.