[RPG] Are Legendary monsters in the Monster Manual ever able to be defeated


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.

My Question:

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?

Best Answer

Using an online Challenge calculator, I am not seeing the issue that you are describing.

Monsters exist up to CR 24, and then the Tarrasque by itself is 30.

For 4 Level 20 PCs, a single enemy of CR 23 is at the high end of a Hard encounter, just barely below the threshold for a Deadly encounter. A CR 24 is thus a Deadly encounter, but 5 Level 20 PCs push it back down to a Hard. (The Tarrasque requires 7 Level 20 PCs to push it under the Deadly level, but lets ignore that one! Using that, you are definitely threatening a TPK!)

I believe you are calculating challenge improperly. I will explain what I think you are seeing, and you can obviously correct me if I am off base.

For the CR calculations(using the table on page 82 of the DMG or page 56 of the Basic DM Rules), a level 20 PC has a Deadly threshold of 12700 XP. A single CR 15 monster is worth 13000 XP. Since that is higher, you see it as being a Deadly encounter. However, for encounter calculations, you must add in the XP threshold for each party member to arrive at your overall thresholds (which is explained in step 2 of the Evaluating Encounter Difficulty section on the same page as the XP Chart). For a party of 4 Level 20 PCs, the Deadly level is set at 50800 XP. A single CR 15 monster against a party of 4 Level 20 PCs will be an Easy encounter.