Phyrexian Altar + Wingrattle Scarecrow + Thran Vigil

Phyrexian Altar

Wingrattle Scarecrow

Thran Vigil

How it works

1. Activate Phyrexian Altar by sacrificing Wingrattle Scarecrow.
2. When Wingrattle Scarecrow 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 Wingrattle Scarecrow.
4. Repeat.

This looping engine relies on Phyrexian Altar to continuously sacrifice Wingrattle Scarecrow for colored mana while simultaneously triggering its persist ability to bring it right back to the battlefield with a -1/-1 counter. To keep this cycle from stalling out due to those accumulating counters, Thran Vigil steps in during your turn by watching the Scarecrow leave the graveyard and placing a neutralizing +1/+1 counter on it; when these opposing counters meet, state-based actions cause them to annihilate each other, leaving the Scarecrow completely clean and ready to be sacrificed all over again.