What can a full level 20 Party take on?

[Zethus]

Title, preferably in CR form with some elaboration. What can 3-6 level 20 characters take on as a very hard challenge?

[MaxWilson]

Tiamat, CR 30.

A trio of Nycaloths and 200 drow, twenty of whom are actually hosts to Intellect Devourers, are likely to also be a challenge. Technically that's a bit over the Deadly threshold even for six PCs, but it would be a hard (and fun) challenge, IMO. Obviously you have to play the drow as intelligent here--don't clump up in easy Fireball formation, play hit and run, use darkvision, etc.

Pretty much anything intelligent will be fun at 20th level, especially if at least some spellcasters are included.

3 Bone Nagas, an Erinyes, a Glabrezu, a coven of three Green Hags, and one Marilith (revised per Dave2008's post here: http://www.enworld.org/forum/showthread.php?482616-Marilith-Battlemaster) are merely Hard, and would also be super fun.

[uraniumrooster]

Three level 20 PCs, if they're well optimized, ought to be able to handle any monster from the MM. Six level 20 PCs, even just partially optimized, ought to be able to steamroll just about anything you throw at them. Level 20 characters are extremely powerful. In my experience, the encounter strength rules in the DMG get less and less useful as PCs advance through the levels, especially if you're running a mid- or high-magic setting and they have accumulated some Rare or better magic items. There are too many variables at that level, and I've seen PCs utterly destroy encounters that are twice the strength of what should be a Deadly fight in just a few rounds, while the same group might struggle with a Medium-difficulty encounter. It depends a lot on the party composition, character builds, and the skill level of your players (but if they made it to 20 I'm assuming they know what they're doing).

For specific CR guidelines, there are some good encounter calculators out there. I like this one (http://dhmholley.co.uk/encounter-calculator-5th/) for the simplicity, and this chart (https://donjon.bin.sh/5e/calc/enc_size.html) can give you a basic overview for encounter size you can throw at them (although it only calculates up to CR 20 monsters, so it's not as useful at high levels).

As a rule of thumb, even middling-CR monsters with spellcasting abilities tend to be able to punch well above their CR, while high-CR melee focused monsters tend to just get more HPs and resistances but don't necessarily pose a great threat to the party. Mixing & matching can turn a Medium encounter into a Deadly encounter if you have your monsters use smart tactics (for instance, have your caster-monster start out invisible and wait a round or two to let the party start focusing on the big meat shield melee-monsters, then when the party thinks they have everything under control, start dropping spells on them).

Also, don't underestimate the Action economy. One big high-CR monster could basically end up as a trivial fight for 6 PCs simply because they can out-Action it, but a horde of low-CR monsters could be extremely challenging.

[Thisguy_]

A 1:1 ratio of Tarrasques to party wizards daily, at least. A CR30 creature which cannot be actually killed without defeating it and THEN making a magical wish.

[Yuroch Kern]

I tend to create "NPC as character" for really crazy challenges. Throwing some of their own abilities and tactics at your players can be terrifying. I agree with the others, 6 level 20's are brutal no matter what. As an avid Planescaper, hit 'em with planer adventures. That'll brutalize anyone if they make one mistake.

[hmjesus]

At that level you're really dealing with a small force of near-gods.

Finding challenges they can't just muscle their way through is often a good idea, but for the sheer combat glory, I'd say you're looking at throwing armies at them. Or complete parties with support (so Tarrasque with a support team of Clerics). Or each other.

[Foxhound438]

if you really want to kill them, just throw save-or-die's until you win.

For a hard fight, I managed to kill a party of 3 in a one-shot last night using an ancient red dragon, a modified pit fiend, and an npc warlock with dominate person. Tons of damage on things with tons of HP is hard to beat.

[R.Shackleford]

Anything except a cavern full of tucker's kobolds.

[Atalas]

Honestly? A full part of level 20 characters can take on pretty much anything. I've had a single level 20 Warlock (Chain, Great Old One) take on a Balor and six Maraliths via just two spells: a Feeblemind on the Balor and then True Polymorph into an Ancient Black Dragon.

[RickAllison]

I just ran a level 20 one-shot with five adventurers!! Here is what I sent against them without care for encounter strength guidelines:

20 Scouts and 60 Shadows in an ambush. Trivialized (as I figured it would be) when the Sun Blade made all the Shadows retreat. Earthquake knocked the goblins out of their trees, the PCs cleaned up. Classic use of player resources to end an encounter without much trouble (though Earthquake might have been overkill...).

1 beefed-up Will o' the Wisp who became invincible while invisible, but only stayed for one turn and then used the next turn for a weak AoE, and 3 Shadows who were monstrosities rather than undead, and could meld into the floor to become invincible for two turns without doing anything. Steamroll, as you would expect. They came, they saw, they conquered.

1 Iron Golem with a demonic shield that drastically uped his AC from the front. His Poison Breath did a number on the party, but it was a cakewalk otherwise.

1 weakened Death Knight with his third attack switched for a half-Dash. They were supposed to beat him, and solved it with an immediate Forcecage. He laughed as they attempted to solve a puzzle, then asked if he should poof away so the prophecy they were following could come true. He was mainly messing with the party, so this should have been easy.

1 large Purple Worm. This was comedic, the cleric botched his knowledge roll so he reported to the party that it was a species that was domesticated for its aphrodisiac venom. Ended up with a little PvP as half the party defended the worm and the other half were trying to kill it. Turning the party against each other made this a more difficult encounter.

And here is where they went off the rails. They were set to go against 12 Wyverns, 5 Mages, and a Fey sorceress whose statblock was suspiciously similar to the Lich. They ended up brokering a deal with her, so I didn't get to see how the fight here or the following part where she turns into a Shadow Dragon would play out. Unfortunately for the party, the final part of the prophecy that was stolen by the Big Bad would have been visible in her dragon chamber, and it told everything they needed to know for the last fight...

The next part had two conflicts. There was a 1v1 with the full-power Death Knight (and with more power than normal...), which the PC was trapped alone for. Fortunately, that PC was built for kiting and he managed to survive just in time. Literally, if his allies had taken one more round to flub, he would have been dead.

Meanwhile, the cleric was smacking the shield barrier, and the rest were off taking out mages and Gladiators with the demonic shields while trying to blow up the shield generators. They ended up blowing up the crystal holding back the world-ending monstrosity.

The final battle was them against the CR 26 monstrosity, Behemoth. I based him off the gnoll demon from OotA, but gave him several interesting features, including only being vulnerable through his horn. The party managed to blow through two of his Legendary Resistance before fleeing with the princess. Making an implacable beast that requires a McGuffin to harm makes for a challenging fight, as you can imagine.

It worked out well enough for them. They all got out alive, they rescued the princess, they killed the evil Paladin. They still left the world-ending abomination and the evil fairy alive, though...

[Foxhound438]

Honestly? A full part of level 20 characters can take on pretty much anything. I've had a single level 20 Warlock (Chain, Great Old One) take on a Balor and six Maraliths via just two spells: a Feeblemind on the Balor and then True Polymorph into an Ancient Black Dragon.

Well, when you're using save-or-dies, your DM can too. A few bad rolls at level 20 can end a campaign.

[MrFahrenheit]

I have an eight player party that Started at 1 and I intend to take all the way to 20. They're 12-13 now, and if I've learned one thing it's this:

Never ever throw an at-or-slightly-above-party-level CR creature against the party as a boss solo. Even in a lair with lair actions. However, lower level minions work wonders, and they can be WAY lower, while still posing a threat, thanks to bounded accuracy. This can honestly make all the difference. Sorcerer or bard roll great initiative and then use their first turn to stun your beholder? Too bad they didn't notice the intellect devourers coming out of a nondescript hole in the wall, or the grick alphas slithering along the ceiling...

[TuesdayTastic]

My campaign is nearing it's end (Players are level 19 and will be level 20 at the end of the session). Just reading this thread is starting to scare me. But here is my suggestion.

You cannot allow them to rest. Ever. Always hound them. If they get the chance to have a full rest, nothing can stop them and they will steamroll everything. If they have a full rest all of their spell slots reset, their abilities, their action surges all come back. Then their damage output is through the roof.

I ran a level 20 oneshot with 4 players. First they had to fight an ancient red dragon (CR 24) and then the Tarrasque (CR 30) one after the other. They destroyed the dragon and then went on to fight the Tarrasque with only 2 players (some had to leave early). They killed the Tarrasque with only 1 player death out of the 2. Level 20 players are scary. This is why you hound them. No rest, no chance to relax. They start to short rest and something new will come up that threatens them. They fight that, move to a better spot and find a new problem that keeps them from resting here. Then you can throw an appropriate challenge rating at them and watch them finally be afraid. Chances are they will still beat that encounter, but the fear will be there.

Just my 2 cp.

[kaoskonfety]

Generally much past level 10 and you need some serious teeth in your threats. My route is to generally make the actual combats not the primary threat, as an example - you are in Hell, literally. The land is awash in hellfire and rivers of rot grubs. Everything is hostile, random encounter 'deer' is replaced with 'some sort of horror that happens to look like a deer but is full of acid that drinks blood'. *Rolls some dice* flaming rocks begin falling from the sky dealing 1d4 fire damage per round - this appears to be what passes for rain here, you hear the rumble of what could be thunder in the distance...