We were discussing with a DM friend of mine about the challenge ratings in D&D 5, and I affirmed the highest known to this day *in the official content* is the [Tarrasque](https://www.dndbeyond.com/monsters/tarrasque), with a CR of 30.

He then said that this wasn't true, and that there was some kind of "mythical" mode that could be enabled and which would include/allow even more deadly creatures. The problem is, he could not remember where he read that.

Did I miss something in the tryptic? Is it possible, RAW, to create monsters with a higher CR than 30?