Doomsday Excruciator, and Phenax, God of Deception

Prerequisites

• Doomsday Excruciator in hand
• Phenax, God of Deception on the battlefield
• You control at least one creature that does not have summoning sickness
• available

Steps

1. Cast Doomsday Excruciator by paying
2. When Doomsday Excruciator enters the battlefield, it triggers, exiling all but the bottom six cards of each library face down
3. Activate a creature you control by tapping it, causing an opponent to mill cards equal to the creature's toughness
4. Repeat step 3 as desired and/or able

Results

• Near-infinite mill
• Near-infinite self-mill