Slayer of Keraptis
I had something similar several years ago playing WFRP 1e. I decided to get into an arena with a minotaur with just my fists. Rolled 5 6's in a row (if you roll a 6 for damage, you roll again and add, and keep adding as long as you keep rolling 6s.)That reminds me of a session, when we played the german "The Dark Eye" back in the days. The game just received a new rules companion "introducing" critical hit tables. Before that the rule was, that if you rolled a 1 or 2 (lower is better here), no parry roll allowed and your damage bypasses armor protection (= no points deducted from the damage roll due to the armor of the enemy (natural and worn alike)). Now there was this fantastic table, that you rolled on after you had that critical.
So during the session, the party encounters this big Hydra. Things go wrong and one of our fighters only had his dagger left. and since he was more or less out (being dragged to the back of the battle with like 2 or 3 HP left), he decides to throw that dagger - To-Hit roll turns a 1 Critical! Now he rolls another d20 - comes up a 20- again - maximum result on the crit table. And now you are allowed to make a third d20 roll to see what exactly happens.And voila another 20 resulting in the immediate death of the beast and party saved. One of the most memorable and cinematic moments in my player "career".