Blightbeetle + Ashnod's Altar + Switcheroo

Blightbeetle

Ashnod's Altar

Switcheroo

How it works

1. Donate Blightbeetle to an opponent
2. Activate Ashnod's Altar by sacrificing the undying creature.
3. When the undying creature dies, its undying ability triggers, returning itself from your graveyard to the battlefield without a +1/+1 counter on it due to Blightbeetle.
4. Repeat from step 2.

This crafty loop transforms a seemingly simple act of charity into a limitless engine by leveraging Switcheroo to hand your opponent a Blightbeetle. Once your opponent is burdened with the beetle's static ability—which strictly prevents any +1/+1 counters from being placed on their creatures—your own sacrifice loop with Ashnod's Altar breaks wide open. Because Blightbeetle suppresses the counter that would normally be placed upon revival, creatures with undying can return to the battlefield endlessly without self-limiting, fueling infinite colorless mana and an unbroken chain of sacrifice triggers.