Mikaeus, the Unhallowed, Phyrexian Altar, and Trapjaw Kelpie

Prerequisites

• Mikaeus, the Unhallowed on the battlefield
• Phyrexian Altar on the battlefield
• Trapjaw Kelpie on the battlefield

Steps

1. Activate Phyrexian Altar 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. Activate Phyrexian Altar by sacrificing Trapjaw Kelpie
4. When Trapjaw Kelpie dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it
5. Repeat

Results

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