Blightbeetle + Phyrexian Altar + Harmless Offering

Blightbeetle

Phyrexian Altar

Harmless Offering

How it works

1. Donate Blightbeetle to an opponent
2. Activate Phyrexian Altar 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. Repeat from step 2.

This clever setup locks down an opponent's mechanics while turning your own graveyard loop into an infinite engine. By casting Harmless Offering, you saddle your opponent with Blightbeetle, whose static ability prevents your opponents' creatures from ever receiving +1/+1 counters. This completely shuts down the safety valve on the creature with undying that you're exploiting, allowing it to return from the graveyard counter-less every single time it hits the dirt. You then repeatedly feed that same creature to Phyrexian Altar to generate infinite colored mana, alongside an endless stream of death, sacrifice, and leave-the-battlefield triggers.