Blightbeetle + Viscera Seer + Bazaar Trader

Blightbeetle

Viscera Seer

Bazaar Trader

How it works

1. Donate Blightbeetle to an opponent
2. Activate Viscera Seer 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. Resolve the Viscera Seer ability.
5. Repeat from step 2.

This clever loop exploits a clever rule interaction: while you normally couldn't sacrifice an undying creature forever because it returns with a +1/+1 counter, having Bazaar Trader donate a Blightbeetle to your opponent completely shuts that down. Because Blightbeetle's passive ability prevents your opponents' creatures from getting +1/+1 counters, your creature keeps coming back clean every single time it dies. With Viscera Seer on hand to instantly sacrifice it and let you scry through your library, you get an endless engine of sacrifices, leaves-the-battlefield triggers, and enters-the-battlefield triggers.