Phyrexian Altar + Thran Vigil + Grazing Kelpie

Phyrexian Altar

Thran Vigil

Grazing Kelpie

How it works

1. Activate Phyrexian Altar 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. Repeat.

This looping engine turns a humble frog beast into an infinite source of mana and triggers by exploiting the interplay between counters and the graveyard. When you feed Grazing Kelpie to Phyrexian Altar, its persist ability brings it back with a -1/-1 counter, while its own sacrifice activation—or the death itself—sends it to the graveyard, simultaneously satisfying Thran Vigil's condition for cards leaving your yard during your turn. That triggers Thran Vigil to place a +1/+1 counter on Grazing Kelpie, which instantly neutralizes the -1/-1 counter through state-based actions and primes the kelpie to persist all over again.