Fraying Sanity, and Fleet Swallower

Prerequisites

• Fraying Sanity on the battlefield
• Fleet Swallower on the battlefield

Steps

1. Declare Fleet Swallower as an attacker
2. When Fleet Swallower attacks, it triggers, causing the player Fraying Sanity is attached to to mill half their library, rounded up
3. At the beginning of your end step, Fraying Sanity triggers, causing that player to mill cards equal to the number of cards put into their graveyard this turn

Results

• Infinite self-mill
• Infinite mill for target opponent