Appa, Steadfast Guardian + Eternal Scourge + Ashnod's Altar + Necrodominance

Appa, Steadfast Guardian

Eternal Scourge

Ashnod's Altar

Necrodominance

How it works

1. Cast Eternal Scourge from exile by paying 3.
2. Appa's second ability triggers, creating a 1/1 Ally creature token.
3. Activate Ashnod's Altar twice by sacrificing Eternal Scourge and the Ally, adding CCCC.
4. Eternal Scourge is exiled instead of being put into your graveyard due to Necrodominance.
5. Repeat.

This ingenious loop uses Eternal Scourge as a recursive sacrifice engine, letting you cast it repeatedly from exile for its standard cost while Ashnod's Altar converts both the Scourge and the resulting tokens into a surplus of colorless mana. Meanwhile, Appa, Steadfast Guardian provides the crucial trigger whenever you cast a spell from exile, populating your battlefield with 1/1 white Ally tokens to feed right back into the Altar. To tie it all together, Necrodominance quietly ensures that Eternal Scourge never hits the graveyard where it could get stuck, instead exiling it whenever it would die so the whole process can spin infinitely anew.