Blightbeetle + Yahenni, Undying Partisan + Bazaar Trader

Blightbeetle

Yahenni, Undying Partisan

Bazaar Trader

How it works

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

This clever loop relies on transferring ownership of Blightbeetle to your opponent via Bazaar Trader, locking them out of ever receiving +1/+1 counters. With that restriction in place, any creature you control with undying will continuously fail to receive its returning counter when it hits the graveyard. This allows Yahenni, Undying Partisan to repeatedly sacrifice that creature for free, growing larger while endlessly churning out death and sacrifice triggers.