is the three nat 20 rule
its very rare to happen and when it does its like omg i have never had any kind of that luck before and is very good time of laughs stories and such when it does things to remember.
role 1rst 20 on d20 crit
role 2nd 20 on d20 max crit damage that can happen
role 3rd 20 on d20 instant death of monster no save no nothing (reason for this is rolling 3 20's in a row is like supper rare i think odds are dont quot me is 1/5k chance)
i like its counter part also three nat 1 rule too
basically the same but in reverse towards the player also just as rare
these two rules are fun when they happen good or bad and make a good game story when they do to make it live for ever.