Creatures CR 50+ (5e Variant Rule)
From D&D Wiki
Creatures Beyond CR 50
If you want to give your characters an extra challenge, these creatures are for characters at least 40th level or so ONLY. A creature CR 51 or more can devastate a whole world, and only the mightiest could stand against them and live. These creatures can be even bigger than Titanic creatures:
Creatures of Massive size occupy a space of 200 ft. by 200 ft. or larger. They have d500 Hit Dice (average 250½).
Creatures of Supermassive size occupy a space of 500 ft. by 500 ft. or larger. They have d1,000 Hit Dice (average 500½).
Monster stats by CR
Since AC 25 means that it is a "very hard" task to hit the monster, and a CR 80 creature deals enough damage to kill another CR 80 with two hits, and the minimum saving throw bonus possible for a CR 80 is +16, which gives the creature a better than 50 percent chance of succeeding on a DC 25 saving throw, the CR track ends here. It is possible for creatures above CR 80 to exist, but 80 is about the maximum CR for creatures that there are more than one of. Creatures more powerful than this are so powerful that even the greatest adventurers could probably not defeat. Anyway, here is the XP reward for CR 51-80:
XP awards by CR
Such powerful creatures can have ability scores above 30, assuming that they are no higher than 50. Here is a list of ability scores and modifiers from 31 to 50: