Blightbeetle + Bloodflow Connoisseur + Switcheroo

Blightbeetle

Bloodflow Connoisseur

Switcheroo

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 hinges on the subtle interaction between a donated Blightbeetle and your own undying creature, whose static ability prevents opponents' creatures from receiving +1/+1 counters. Because Blightbeetle's effect applies to your opponent (who now owns the beetle), the undying creature you previously gave away—or rather, the sacrifice loop enabled by Bloodflow Connoisseur—gets neatly bypassed: when your sacrificed creature returns via undying, the beetle stops the counter from being placed. This allows you to continually sacrifice the creature to Bloodflow Connoisseur to grow it infinitely while triggering endless sacrifices and deaths. Finally, Switcheroo is the magic wand that kicks off the engine by giving that crucial Blightbeetle to your opponent in the first place, locking them out of counter-based safety nets while you reap the rewards.