Agatha's Soul Cauldron + Walking Ballista + Butcher Ghoul

Agatha's Soul Cauldron

Walking Ballista

Butcher Ghoul

How it works

1. Activate Butcher Ghoul using Walking Ballista's second ability by removing a +1/+1 counter from it.
2. Repeat step 1 until Butcher Ghoul has no +1/+1 counters on it.
3. Resolve all Butcher Ghoul abilities dealing 1 damage to Butcher Ghoul, and any remaining damage to any target(s).
4. Butcher Ghoul dies as a state-based action due to having 0 toughness.
5. Butcher Ghoul's undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
6. Repeat.

This clever loop relies on Agatha's Soul Cauldron to bridge the gap between our engine pieces, granting Walking Ballista the activated damage-dealing ability of Butcher Ghoul while keeping any color of mana flowing. By using that borrowed ability to repeatedly remove counters, we ping Butcher Ghoul to death and drain our opponents simultaneously. Once the ghoul hits the graveyard, its undying ability brings it right back to the battlefield with a fresh +1/+1 counter, resetting the whole cycle for infinite death triggers and damage.