Riverchurn Monument, and Lord Xander, the Collector

Prerequisites

• Riverchurn Monument on the battlefield
• Lord Xander, the Collector on the battlefield
• available

Steps

1. Declare Lord Xander as an attacker, triggering its ability, causing an opponent to mill half their library, rounded down
2. Activate Riverchurn Monument's exhaust ability by paying and tapping it, causing each player to mill cards equal to the number of cards in their graveyard

Results

• Infinite mill for target opponent