Mikaeus, the Unhallowed, Altar of Dementia, and Trapjaw Kelpie

Prerequisites

• Mikaeus, the Unhallowed on the battlefield
• Altar of Dementia on the battlefield
• Trapjaw Kelpie on the battlefield

Steps

1. Activate Altar of Dementia by sacrificing Trapjaw Kelpie
2. When Trapjaw Kelpie dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it
3. Resolve the Altar of Dementia ability
4. Activate Altar of Dementia by sacrificing Trapjaw Kelpie
5. When Trapjaw Kelpie dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it
6. Resolve the Altar of Dementia ability
7. Repeat

Results

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