Mikaeus, the Unhallowed, Phyrexian Altar, and Heartmender

Prerequisites

• Mikaeus, the Unhallowed on the battlefield
• Phyrexian Altar on the battlefield
• Heartmender on the battlefield

Steps

1. Activate Phyrexian Altar by sacrificing Heartmender
2. When Heartmender 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 Heartmender
4. When Heartmender 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