Mikaeus, the Unhallowed, Twilight Shepherd, and Phantom Train

Prerequisites

• Mikaeus, the Unhallowed on the battlefield
• Twilight Shepherd on the battlefield
• Phantom Train on the battlefield

Steps

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

Results

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