I'm starting my first 5e campaign soon and I'm just learning about balancing encounters. While looking up challenge ratings I noticed a very troubling truth:
Of 30 challenge levels in the Monster Manual, more than half of the monster ratings (all monsters CR 15-30) are considered beyond deadly for a party of four at max level, by themselves. According to DMG 82 on XP thresholds, 12,700 XP encounters are deadly for 20th level parties. All monsters at CR 15 or higher begin at 13,000 XP. In fact, by these calculations a Tarrasque is more than 10 times as deadly as any encounter that's expected to be lethal to a level 20 party, and its CR is twice what is considered rational for encounter planning. Dragons, Liches, and everything else between aren't much less daunting.
It's bad enough to have to wait for my party to reach double-digit levels before they take on Legendary monsters. But the fact that I can't expect to use one—ever—without a TPK just seems a little disheartening.
Why do CR 15-30 monsters exist if they're designed to outright kill a party of any level with no contest? Is it a joke? It this just for GMs who hate their players? Or is there some secret loophole I'm missing that makes victory against Legendary monsters feasible?