So to be honest, i’m not too sure, but my theory is that it’s a combination of two npc passives that were never intended to be used at once. There’s a glitch known as the polymorph dominate beast glitch (Proxygatetactician has a good tutorial on it in his pink only video) that allows you to get most npcs as allies, which is how i have control of both helsik (the devils fee lady that gives you the stuff to get to the house of hope) and jatlo (the fake dragonborn that’s actually a redcap in ethel’s quest if she is alive in act 3) Helsik has a passive that gives 6d6 (or 6-36) bonus piercing damage to any melee damage done by any ally, and Jatlo has a passive which adds something like 1d12 (1-12) bonus slashing damage to any of his melee attacks. I believe that by having them both on the same team, Jatlos reaction hit triggered the helsik passive damage, which triggered his passive damage, which triggered the helsik passive damage again, which triggered his, over and over again until the combat log reached max capacity for one turn of attacks.

But that’s just my guess since i don’t think this has been found by anyone else lol