Sepulchral Primordial + Infinite Reflection + Altar of Dementia

Prerequisites

• You have a way to cause a nontoken creature to enter the battlefield under your control.
• An opponent has a creature card in their graveyard.

Steps

1. 1Cause a nontoken creature to enter the battlefield under your control, which enters as a copy of Sepulchral Primordial due to Infinite Reflection.
2. 2When the creature enters, it triggers. Holding priority, activate Altar of Dementia by sacrificing the creature.
3. 3Resolve the creature's trigger, putting a creature card from an opponent's graveyard onto the battlefield under your control as a copy of Sepulchral Primordial.
4. 4Repeat from step 2.

Effects

• Infinite creature ETB
• Infinite creature LTB
• Infinite creature sacrifice triggers
• Infinite death triggers
• Infinite mill
• Infinite self-mill