Mikaeus, the Unhallowed, Trapjaw Kelpie, and Carrion Feeder

Prerequisites

• Mikaeus, the Unhallowed on the battlefield
• Trapjaw Kelpie on the battlefield
• Carrion Feeder on the battlefield

Steps

1. Activate Carrion Feeder 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 Carrion Feeder ability
4. Activate Carrion Feeder 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 Carrion Feeder ability
7. Repeat

Results

• Infinite +1/+1 counters on a creature
• Infinite ETB
• Infinite LTB
• Infinite death triggers
• Infinite sacrifice triggers