Thran Vigil + Grazing Kelpie + Phantom Train

Thran Vigil

Grazing Kelpie

Phantom Train

How it works

1. Activate Phantom Train by sacrificing Grazing Kelpie.
2. When Grazing Kelpie dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Thran Vigil triggers, putting a +1/+1 counter on Grazing Kelpie.
4. Resolve the Phantom Train ability.
5. Repeat.

This clever loop relies on Phantom Train acting as a repeatable sacrifice outlet that simultaneously satisfies Grazing Kelpie's persist trigger and provides a vessel to grow. When Grazing Kelpie dies, it returns to the battlefield with a -1/-1 counter, but Thran Vigil notices it leaving the graveyard and places a neutralizing +1/+1 counter right back on it. State-based actions then cause the +1/+1 and -1/+1 counters to annihilate each other, resetting the Kelpie so its persist ability can trigger all over again.