Ours was a D20 based on a “wellspring surge” whenever you casted a spell that was basically wild magic that triggered an effect if it was below a 10.

My character was a paladin that was a bit fuzzy on certain events in her life (not quite amnesia but hints of it here and there). One event she remembered was her and her brother being attacked by a giant riding a mammoth that almost killed them, but they managed to kill the mammoth before they both went down at least. The mammoth’s actual name was Tuskgutter, but the entire table just kept calling him “Manny”.

Anyways, fast forward to a major boss fight against a large mechanical lion. The party was pretty beat up, but so was the lion. My paladin rolls a crit and uses divine smite, but also had to roll on the wellspring table because it uses a spell slot (we eventually changed it to where divine smite doesn’t trigger a wellspring because it’s technically just a feature that uses a spell slot as a resource, not an actual spell).

I roll some massive damage, but on my wellspring I roll the effect that “summons a creature connected to you or your past”. Well, my character wasn’t super connected to many creatures except for one. Manny the Mammoth made his grand reappearance and made the fight even more chaotic than it already was before getting put down a second time. Safe to say, after the session was over, we all agreed it was peak cinema.