Blightbeetle + Bloodflow Connoisseur + Bazaar Trader

Blightbeetle

Bloodflow Connoisseur

Bazaar Trader

How it works

1. Donate Blightbeetle to an opponent
2. Activate Bloodflow Connoisseur 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 Bloodflow Connoisseur ability.
5. Repeat from step 2.

This clever loop relies on a subtle interaction with Blightbeetle's static ability, which prevents your opponents' creatures from ever receiving +1/+1 counters. By using Bazaar Trader to hand that very same Blightbeetle over to your opponent, any creature with undying that you control will continually return to the battlefield counter-free when it dies. From there, you simply feed that creature to Bloodflow Connoisseur to generate infinite death triggers, enter-the-battlefield triggers, and endless +1/+1 counters for the Connoisseur.