Anti-Venom, Horrifying Healer + Agatha's Soul Cauldron + Furnace of Rath + Walking Ballista

Anti-Venom, Horrifying Healer

Agatha's Soul Cauldron

Furnace of Rath

Walking Ballista

How it works

1. Activate Anti-Venom using Walking Ballista's ability by removing a +1/+1 counter from it, targeting itself and putting two +1/+1 counters on it due to its replacement effect and Furnace of Rath.
2. Activate Anti-Venom using Walking Ballista's ability by removing a +1/+1 counter from it, dealing 1 damage to any target.
3. Repeat.

This intricate loop relies on Agatha's Soul Cauldron to grant Walking Ballista the activated abilities of Anti-Venom, Horrifying Healer while it sits in exile or on the battlefield with a +1/+1 counter. Once that synergy is established, Walking Ballista can tap its newly acquired ability to ping itself for damage, which Furnace of Rath doubles before Anti-Venom's replacement effect prevents it and converts it into a net gain of +1/+1 counters. Because you are generating more counters than you remove to pay the activation cost, you can endlessly repeat the process to pile infinite counters onto your creatures and deal infinite damage to any target.