Mikaeus, the Unhallowed, Phyrexian Altar, and Wingrattle Scarecrow

Prerequisites

• Mikaeus, the Unhallowed on the battlefield
• Phyrexian Altar on the battlefield
• Wingrattle Scarecrow on the battlefield

Steps

1. Activate Phyrexian Altar by sacrificing Wingrattle Scarecrow
2. When Wingrattle Scarecrow dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it
3. Activate Phyrexian Altar by sacrificing Wingrattle Scarecrow
4. When Wingrattle Scarecrow dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it
5. Repeat

Results

• Infinite ETB
• Infinite LTB
• Infinite colored mana
• Infinite death triggers
• Infinite sacrifice triggers