Blightbeetle + Harmless Offering + Yahenni, Undying Partisan

Blightbeetle

Harmless Offering

Yahenni, Undying Partisan

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 delightfully twisted loop turns a seemingly defensive insect into an engine of endless doom by handing it to the other side of the table using Harmless Offering. Once Blightbeetle happily resides under your opponent's control, its static ability ensures that any creature they control can never receive +1/+1 counters. When you feed your own looping undying threat into Yahenni, Undying Partisan's sacrificial altar, that prevention clause stops the usual counter from returning with it. Without that counter to lock the creature out of future returns, the cycle can repeat infinitely, generating endless sacrifices, enters-the-battlefield events, and leaves-the-battlefield triggers while growing Yahenni to monstrous proportions.