Riverchurn Monument, and Fleet Swallower

Prerequisites

• Riverchurn Monument on the battlefield
• Fleet Swallower on the battlefield
• Fleet Swallower does not have summoning sickness
• You have not activated Riverchurn Monument's exhaust ability
• available

Steps

1. Declare Fleet Swallower as an attacker, triggering its ability, causing a player to mill half their library, rounded up
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, thus causing the player targeted by Fleet Swallower's ability to mill the remainder of their library

Results

• Infinite self-mill
• Infinite mill for target opponent