Need Ideas for a Level 20 Oneshot

So, I had the briliant idea to propose to run a level 20 oneshot for some of my friends. But it turns out, level 20 D&D is really hard to prep. 

So, I managed to created the introduction where the players will face a couple of puzzles and some light combat where they can test their new abilities a bit out before confronting the BBEG. Now the problem is, I don't have a BBEG yet...

So if anyone has any good suggestions for a BBEG that I can use against 5 lvl 20 characters, or knows of any good published (official or 3rd party) level 20 oneshots that I can look to for inspiration, please share them. I would love to hear all of your ideas.

if you are really bored, give them a valor bard warlock. conjure minor elementals+eldritch blast. (chefs kiss)

Sounds fun, but unfortunately we agreed to use the 2014 rules only. So I might just have to use a gloomstalker/rogue/fighter multiclass instead ;)

Quote from EarthSword28 >>

Sounds fun, but unfortunately we agreed to use the 2014 rules only. So I might just have to use a gloomstalker/rogue/fighter multiclass instead ;)

 sorry I don't know much 2014.

A Solar that has been enslaved by a Pit Fiend so is compelled to attack but constantly apologizes for doing so and begs the players to free it. It's Pit Fiend master, plus 2-3 Iron Golems made of Infernal Iron by the Pit Fiend as its personal bodyguard. 

Some sort of incursion from another plane is usually the most appropriate thing for level 20, but dragons can work too.

Some ideas:

• Aberrations spread through a village and harry the PCs while cultists summon an eldritch entity to destroy the world. They've got to figure out what the cultists are planning, find the ritual site and get past the tricksy traps and puzzles guarding it, and OHNO! It's too late. Fight a few Cosmic Horrors or a reskinned Kraken.
• Similar thing but with a demon lord instead of aberrations.
• A mechanical dragon flies over the characters in the wilderness towards the city they're also traveling to, and the players can try to fight it. If they do, it's actually really easy, but the true threat is the ticking bomb hidden in the sewers of the city. A desperate sewer dweller enlists their aid in defeating clockwork monstrosity invading the sewers, and in the course of that the players find the bomb, which is inside of a mobile mechanic suit that they have to race against time to defeat and dissarm.

Ooh, I really like those ideas. I think I'll go ahead and give the Invasion a try.

Thanks a lot.

Try something that gets lots of attacks but give them teleportation so ranger/rouge with a spell caster. Also add minions 

Fires of Iskh on Arcane Library is quite good.

Action economy, action economy, action economy. Most single monster encounters are a joke at level 20, largely because even the strongest monsters get eclipsed in action economy by the team. Even a Tarrasque only gets eight total actions (four attacks with multiattack, a swallow, and three legendary actions), while a Conjuration Wizard (UA content, I know, but still) could summon two Draconic Spirits with ten total actions between the two of them, and I'm not even being creative with my thinking. If you're doing a BBEG, they CANNOT beat the party on their own, I don't care HOW powerful they are. Surround them with minions, create crystals that shoot lasers, have the skies open up and ******* HORNETS swarm the party, whatever you need to do to match action economy. A party Fighter on a good round can get anywhere from eight to twelve attacks off, a Cleric can clean-sweep a room with a handful of spells, a Wizard can summon ******* meteors. So make sure to force the players to waste their resources on something other than your BBEG, or you're cooked.

Also, 20th-level needs a lot of dynamism in its challenges. Consider that the players have a variety of movement options available to them, so don't be afraid to think in three dimensions. Challenges in the sky, underwater, or on terrain that is constantly shattering, reforming, or crashing into itself are on the table. Environmental effects like gas clouds, lava eruptions, and sudden shifts in gravity are always great. Really bend your mind.

The reason 20th level is the last level, is because you're usually learning how to run progressively more complex and challenging combat from level one to level 20. Since you haven't done that, you've basically signed yourself up for a crash course in doing the most complex and hardest game of your life. Don't be afraid to get weird. Also, don't be afraid to cheat. I know it sounds crass, but you are at a disadvantage here, there's no way you'll know what tricks your players will have up their sleeves. If they do 800 damage to your BBEG in the first round of combat, give the BBEG extra hp. Give them a reaction that reduces damage. Give them a ******* shield spell for chrissakes. If there are too many monsters, reduce their hp, or have the wizard's Fireball obliterate more of them than it should. Bend the rules to keep the game moving, keep the players engaged, and keep the story on track. If you were a DM who built a team from level one to twenty over years of play, I wouldn't say to do this, but you're not, so just... fake it as best as you can.

When playing with level 20 players, your primary goal needs to be draining their resources at all times.  If they get to the big bad with all their resources intact, either you will have an unsatisfying fight due to brevity, or the fight will stretch out for an incredibly long time to ensure the players feel an appropriate sense of dread.  Neither is ideal.

To that end, puzzles should be dealing significant amount of damage on fail states.  There should be no "light combat" - every combat you run should involve dangerous monsters that risk killing the party if the party does not spend some of their spell slots and ability points on killing them.  Generally, I think it plays better to focus on a number of lower HP, extremely high damage monsters than solo fights at this level - requires more thought to kill, and you neither can nuke a single creature nor ignore using resources on everyone, as any given creature can take a chunk out of the party's health. 

Your final fight, regardless of what the BBEG is, should also be dynamic.  Terrain that has to be navigated.  Other very powerful monsters  A BBEG that can move around easily so it is not just a tank and spank.  Player characters at that level have a lot of options - you need to keep your bad guy equally dynamic to keep up.  Be sure to remember lair actions and legendary actions, and do not be afraid to give them more legendary action points to spend per round.  No official monster does that very well, so it might be a good idea to look at several CR 30+ monsters and borrow abilities from all of them.

Thank you all so much for the amazing ideas, you've all been so helpful and I will definitely be making use of all of your advice.

The one shot came about when me and my friends were talking about an old campaign we were all playing in but fell apart shortly before we reached level 15. So I offered to run this one shot so that everyone could play their character one final time but this time actually make it to level 20 like the original plan was. 

Before they'll face the BBEG they'll have to fight one of the previous bosses we actually defeated during the original campaign. At the time it was the hardest fight we had fought yet and not all of us walked away from it. (We lost the only person with revivify in the party. 😅) But now I expect they'll just destroy that miniboss, though I also hope they'll use some recourses doing so. 

As for the BBEG themselves, I've decided to just let them fight a deity in a temple built on the side of a (now no longer) dormant volcano. Please be mindful of the canals, the water may have evaporated but that doesn't mean they are empty now.

I am currently homebrewing something fun and I will definitely be posting it when it is done.

Quote from RWinnie >>

Action economy, action economy, action economy. Most single monster encounters are a joke at level 20, largely because even the strongest monsters get eclipsed in action economy by the team.

It's not action economy. It's just raw numbers. The tarrasque does around 250 (base) damage per round and has 676 hp. A party of 4 level 20s does has around 400 (base) damage per round and, with healing and other abilities, takes around 1500 damage to drop.

BEHOLDER SWARM!

Quote from Pantagruel666 >>

Quote from RWinnie >>

Action economy, action economy, action economy. Most single monster encounters are a joke at level 20, largely because even the strongest monsters get eclipsed in action economy by the team.

It's not action economy. It's just raw numbers. The tarrasque does around 250 (base) damage per round and has 676 hp. A party of 4 level 20s does has around 400 (base) damage per round and, with healing and other abilities, takes around 1500 damage to drop.

 
I would say these are two sides of the same coin - the reason a Tarrasque cannot perform as a solo monster is its action economy does not match what players can dish out.  One of the ways I fix this is to remove some of the action economy limitations that were clearly designed without an eye toward what players actually can do at high levels.  

Using the Tarrasque as an example, with the idea of making it the solo monster it is legendary for, the first thing you have to do is increase hit points drastically.  Then I start improving the action economy on the stat block.  During its turn, it normally has to choose between breath or four attack, making the breath option an action economy problem.  I likely would say that the Tarrasque always makes four attacks, one of which can be a breath weapon if it is recharged.  Next, I would change its legendary action that only provides one attack to allow multi attacking - the number of attack to be determined by how many attacks the party gets and whether they have a full healer in the group. Finally, I likely would add lair/environmental actions - falling trees, tremors, panicking animals darting from the underbrush and making attacks, etc.  Things that would change up the battlefield to both show the size of the fight and make it more dynamic than solo fights against large animals tend to be.  In

All of this gives the monster more actions and thus more damage.  Probably still not my favorite fight - this is the kind of thing I would run as a wilderness encounter or mini boss at level 20 - but the general idea holds true for most level 20 encounter designs.  Improving action economy by removing “either or” choices that punish you for using cool abilities helps a lot, since you can keep the action economy going while still using those signature moves that make the monster unique.  Changing legendary actions to work more like a full action better matches your players - whether that is by increasing the number of attacks listed above, or, for things like Spellcasters, removing restrictions like “third level or lower” from the spells they can cast as a legendary action.

Lair actions give you another fun way to make a signature set of abilities specific to the boss, while also allowing more status abilities or effects. Puzzles are a fun way to win with lair actions for intelligent monsters.  A puzzle that does bad things on the lair action initiative if not solved can do things like damage or bad status effects, but more importantly requires players to spend their action economy mid-combat to solve it, fixing some of the damage and economy gap. 

And, as I said before, more monsters can also help - particularly if they are things that require an answer rather than just piling on the bad guy (ex. The bad guy can use them to heal, or the additional monster can put out a terrifying effect like Finger of Death that might turn an ally into an enemy zombie).

All of those are things that can help.  How much or how little you do is going to depend on what your specific party is.  When I run games, players are usually pretty good about having a couple healers because they know the games will be brutal, so I often take an “all of the above” approach.  But that will not work for every party.  Fortunately, you are doing a one shot, so you can take a little more risk - even if you do some resource drain along the way, they might be more full on resources when they hit the boss.

Plus, you can alway start smaller and have the boss entered a “frenzied” stage when bloodied, adding more abilities/better action economy to adjust for a lack of power in the first half of the fight. 

Quote from Caerwyn_Glyndwr >>

 I would say these are two sides of the same coin - the reason a Tarrasque cannot perform as a solo monster is its action economy does not match what players can dish out. 

 Action economy, outside of status effects that remove actions, is just how you divide up your available power budget. Just increase the Tarrasque's hit points and damage per attack by 50-100%, with no changes to action economy, and as long as it's able to engage the PCs in the first place (it's always been a poor boss monster due to not having good ways of dealing with long range bombardment) it's a legitimate threat.

I wrote up my own Tarrasque variant that generally should be a better boss fight, though it's not really tuned for level 20 even so: https://www.dndbeyond.com/monsters/5794084-mythic-tarrasque