What is the lowest-level party that can go against a CR 20+ and have fun?

[Notafish]

I'm curious about your experiences with "unbalanced" encounters, and the feasibility of having a game with a low level cap that nevertheless features ancient dragons and the like as potential antagonists. At what point can a party get a feeling of being severely out of their league, but not automatically feel the need to retreat or call for back-up? Are magical weapons mandatory? What (non-cheese) tactics could a mid-tier party use versus an ancient dragon or a lich?

[TyGuy]

I'm curious about your experiences with "unbalanced" encounters, and the feasibility of having a game with a low level cap that nevertheless features ancient dragons and the like as potential antagonists. At what point can a party get a feeling of being severely out of their league, but not automatically feel the need to retreat or call for back-up? Are magical weapons mandatory? What (non-cheese) tactics could a mid-tier party use versus an ancient dragon or a lich?

Level 1 with the right people and setup.

Otherwise let's look at what it takes to survive against an ancient white. Looking at a breath weapon with average damage of 72/36. Let's say the team has ample intel and preparation. And they have resistance potions. So that average 72/36 is now 36/18. A d10 class with +2 con is looking at 36 hp at level 4. There's also things like aid and temp hp features, but for now I'm going to look at this as the level where, again with preparation like resistances, the party isn't going down instantly with the first breath weapon. Characters will probably still get merced quickly by attacks and legendary actions, but at least level 4ish is where the party is getting out of the "rocks fall, you die" instant gib territory.

[Witty Username]

I have been wanting to put together a lv1, one-shot to that effect. Castle attacked by a dragon, the goal of the players to survive. Maybe they sneak out, maybe they get the batista going, maybe they bargain with it. Direct combat is the thing they likely can't do.

[Notafish]

Thanks for the replies - Witty, I have also been interested in doing something along those lines, either as a DM or as a player. For whatever reason, changing max HP every level is frustrating to me - possibly because when I play pencil and paper, the HP section of my character sheet gets illegibly smudged far faster than anything else...

After considering TyGuy's reasoning process, I also started to wonder whether I could make a level-less 5e variant with player character HP equal to STR+DEX+CON, and the mental stats contributing to something magic or inspiration-related. it would be a shift from a standard game, but maybe viable and somewhat compatible with the Monster Manual?

[sithlordnergal]

It heavily depends on party composition, player skill, and what type of enemy you're facing. For example, when I played in ToA, our party was about level 10. The Moon Druid cheesed the final boss by dragging it into lava as a Fire Elemental, killing it in 3 rounds.

Acererack then showed up, and the party managed to beat him. Normally you're supposed to have a bunch of buffs as you fight him, advantage on all attacks and a ton of temp HP that refreshes each round. We did not have that. But our party won anyway.

Cut to a different group and they can barely handle a CR 17, let alone a CR 20

[Guy Lombard-O]

Maybe they sneak out, maybe they get the batista going, maybe they bargain with it. Direct combat is the thing they likely can't do.

I don't know. If they're a group that has a spell castro, they might well decide that direct combat is the way to go instead of bargaining...?

[J-H]

I would say somewhere between level 10 and 13, depending on:
-# of players
-tactical competency of players
-specific builds
-frequency or dearth of magic items

Using the BlogOfHolding method, a typical CR 20 has AC 19 and 300 HP, and puts out about 100 damage per round.
I'd put my 10th level Castle Dracula party (6, frequent magic items) up against most CR20s and say they'd win pretty handily. In some boss fights they were dumping 150+ damage per round thanks to crits, smites, GWM, etc.

[Sigreid]

The easiest way to make it possible without fudging would be to let the early quests be for the gear to defeat the dragon. Armor of resistance, scrolls that can help, weapons of dragon slaying, etc. Make them real quests with real risks and full on parts of the campaign.

[Witty Username]

I don't know. If they're a group that has a spell castro, they might well decide that direct combat is the way to go instead of bargaining...?

If they want to try let them, If a level 1 party goes toe to toe with an ancient dragon and win, I would definitely want to be there to see it. I just expect that is the approach that is least likely for a level 1 party to succeed at or be prepared for.

[Dork_Forge]

Probably early Tier 3 depending on the experience of the players and magic items available. Anything before that is inviting a swift death.

[KorvinStarmast]

We had a party of 4 level 11 PCs go up against a lich.
CR 21.
The lich was defeated, although the blade lock died to PWK (revivify for the win, cast by the paladin!).
We did not, however, figure out where her phylactery was. When she reformed near it we started getting obscene sendings from her with some frequency. Yes, she wanted a rematch, and it took us a while but she eventually got one. IIRC, we were level 13 at that point, and it didn't go well for her.