Agatha's Soul Cauldron + Walking Ballista + Strangleroot Geist

Agatha's Soul Cauldron

Walking Ballista

Strangleroot Geist

How it works

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

This brilliant loop relies on Agatha's Soul Cauldron to grant Walking Ballista's activated ability—specifically the ability to remove a +1/+1 counter and deal 1 damage—to Strangleroot Geist, which conveniently arrives with its own haste. Because Agatha's Soul Cauldron also lets you spend mana as though it were any color and feeds on graveyards, the engine is fully unlocked. By continuously removing counters from Strangleroot Geist to ping itself, you eventually reduce its toughness to zero, sending it to the graveyard. This death gracefully triggers Strangleroot Geist's undying ability, bringing it right back to the battlefield with a fresh +1/+1 counter so you can repeat the process indefinitely for an endless stream of entries, leaves, and demises.