Mikaeus, the Unhallowed + Phyrexian Altar + Thunderblust

Mikaeus, the Unhallowed

Phyrexian Altar

Thunderblust

How it works

1. Activate Phyrexian Altar by sacrificing Thunderblust.
2. When Thunderblust dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Activate Phyrexian Altar by sacrificing Thunderblust.
4. When Thunderblust dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
5. Repeat.

This loop is a masterclass in counter cancellation, using Mikaeus, the Unhallowed to grant undying to a creature that naturally possesses persist. When Thunderblust hits the graveyard for the first time, its persist ability brings it back with a -1/-1 counter, but its next death triggers undying instead, returning it with a +1/+1 counter which immediately annihilates the lingering -1/-1 counter through state-based actions. Meanwhile, Phyrexian Altar provides both the sacrifice outlet needed to trigger these cyclical deaths and the mana generation to fuel the engine, resulting in an infinite supply of colored mana, sacrifices, and death triggers.