Something to consider for adventuring beyond 20: The PCs are some of the most powerful entities in existence. They should not face many enemies that pose a large risk to their lives. If they do, you diminish how powerful they are. They're Supermen. And like Superman, the way to challenge them is not by threatening their lives, it is by threatening the things they care about. Threats to their loved ones, their kingdom, their faith... these all the PCs to be stronger than others, but still face challenges.
I haven't played high level games yet but I see a lot of monsters with CR's over 20. Do characters plough through these as easily as you suggest?
Can I just say something? Players have fun where they are challenged and aren't limited by the imagination or desires of the DM. Most DMs are intimidated by high level characters, and in the case of Immortals, terrified. They don't want to look inept or incompetent having put forth challenges, only to see the characters easily overcome them or having looked foolish from the attempt.
It's called Dungeons and Dragons, but Dungeons can only go so deep and Dragons can only be so tough, until they become something of an annoyance as opposed to any real challenge.
That leads DMs changing the rules to make it harder. Doubling monster's HP, increasing the frequencies or even adding strange and bizarre powers and defences to existing powers.
And when all of that fails, just simply say, "Oh you find your spells no longer work" and that 30th level Wizard becomes zombie food.
I personally think high level PC's and Immortals have a place in adventuring, if DM's think outside the box. Going to other planes of existence to deal with some ancient evil attempting to open a portal to the Prime. Go to one of the vast Galaxies to be involved in an interstellar rebellion against an extremely powerful Empire that rules dozens of worlds. Perhaps uncovering the machinations of one or more evil Immortals or Fiends.
The possibilities are endless.
Why is there an assumed stopping place of level 20 in 5e?
- Nobody plays that long?
The fact that 5E campaigns still peter out before hitting 20th is one of the things that makes me suspect it isn't about power level at all, per se--it's about DMs who haven't prepared for what a level 20 campaign would or should look like.
Tradition? I think that's where it stopped for every edition but 4e?
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(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.