D&D 5E Creating a Challenging monster for a Lvl 20 party

[Voi_D_ragon]

So, I've been thinking. Most high CR monsters are designed around the idea that they should be played intelligently (which is great, since hitting bags of HPs is boring anyway). I see this especially in dragons and beholders, but that's probably just because they're the only ones I've given any thought to.
The only high CR monster I see as just being a bunch of HPs with attacks is the Tarrasque, and, apart from the fact that it's so iconic it's almost heresy to actually present it in play, it's also the most popular protagonist of theorycrafting-
"Can a party of 20 beat the Tarrasque?"
"Yes, you poor little fool. There are rumors regarding a 2 man combo (lvl 20) that can bring the fell beast down. Or just a commoner with a candle of invocation that calls a Solar down to help him"

Plus, the Tarrasque has no nuance. It hits you, and if you're still there, it will hit you again. Then you hit it, and if it's still there afterwards, it will hit you back. Rinse, repeat, epic boss fight...?
That being said, this is all from looking at its stats and hearsay (and, yes, I have read about the spellcasting, army-leading Tarrasque and thought that was really awesome), so I don't know how it really plays, but it doesn't tickle me. (Yeah, you have to put the fight into context -maybe he's destroying a city and you have to kill him quickly or without causing too much collateral or something-, but you're still punching each other until someone dies)

So. Long preface, I know. Anyway, I was thinking, is there an actual way to make a monster that doesn't fly, doesn't use its lair/environment to gain tactical advantages, and basically just sits there being a good ol' bag o' hit points in a way that he is an actual menace to a party of lvl 20 adventurers at the peak of their abilities -just woke up from a long rest, count any magic items/resources the group itself can bring, not any allies-?

The monster can (well, must, probably, to make the fight in some way dynamic) be able to cast spells, and, to be able to resist more than a couple of rounds against the party, is probably deity-level in power overall.

However, the monster is very confused/not particularly bright and doesn't like positioning, so it fight slugfest style, only better (I might need to reword that better, but you should, no zippget what I mean, no zipping around here and there trying not to get hit by anything)

Some things I was considering
-condition immunities (oh so many of them) or maybe something to make them not so crippling (such as reducing any lasting effects to 1 round) (this is on top of legendary resistance, clearly)
-long range attacks (loooong range, 300 + ft)
-some way to deal with fliers (be it with these long range attacks or otherwise)
-properties that make it dangerous all round long (I think of the balor's burning aura, along with, of course legendary actions, but cranked up to insane levels)
-various levels of danger ("this isn't even my final form!", or some power source that can be destroyed to weaken him, but can't be done if not while confronting him -and thus turning attention away from him for a few rounds-)

I might come up with other things while this is up, and I'll edit them, but I have other things to post rn, so I'll toss you guys this thing while I work on the rest. (More posts on the way )

 

[Oofta]

I did something like this (although I ended up not using it). Basically a Tarrasque crossed with a phase spider (my inspiration was a big plastic spider). Unfortunately somehow I've lost the details so going from memory here.

I don't usually do solos, and this one is kind-of-sort-of a solo

Some of the things I tweaked:
I kept things like the reflecting spells.
I gave the spider "minions". The exterior of the spider was constantly shifting, it was actually crawling with thousands of "baby" spiders which shook off every other turn casting off swarms (mobs actually) of giant phasing spiders. I could use this to attack people anywhere - the idea was they could phase in to a flying PCs spot, grappllng and pulling them down.
The spider could also shoot webs, restraining and pulling victims in close
The spider could phase, which I could also use as legendary-resistance type actions to get out of effects.

After the spider dropped to 2/3 of it's HP, it's shell started to crack.
AC dropped
The spell reflection percentage dropped, but attacks sent out spurts of poisonous/acidic blood.
In addition it gained a new legendary action; it could "stomp" which caused echoes through the ethereal plane and did sonic damage and knock people prone.

After the spider dropped to 1/3 of it's HP, it's shell cracked more
AC drops again
There was no more spell reflection
It started screaming in agony causing an aura of psychic damage
As a reaction after every PC's turn it phased to a different location, ending any ongoing spell.

So that was one option. Big brute (with minion mobs to help), minor transformations that changed the way the creature played. The details don't matter a huge amount, but it wasn't a true solo, had some transformations and changed over the course of the game. Oh, I modified the spider so they didn't do poison but instead did Acid damage, but that was just a preference (poisoned condition is annoying).

 

[Eltab]

- BBEG is actually Small size, and some of his toughness is due to the fact that he can make you come after him in single file down a narrow hallway.
- BBEG is big and strong enough to turn any sort of building into a throwing weapon - such as a cottage/bomb or a tower/javelin
- BBEG creates his own terrain, which hinders melee-artist PCs, and doesn't mind retreating back a few steps sometimes.
- BBEG has an aura that makes PCs want to not get TOO close. This could range from Whispers of Suggestion/Enthrall to an infectious disease to Skunk Odor.
- BBEG is insubstantial and just walks through walls if the PCs try to hide in the next room
- BBEG can Rage like a barbarian, to get more mileage out of his HP supply
- BBEG has a Sneak Attack -like Nova capability
- BBEG has already prepped the battlefield with spells like Permanent Guards and Wards before the PCs arrive. (The longer they take to get there, the thicker his preparations.)
- BBEG has already prepped the battlefield with traps and other mechanical hindrances before the PCs arrive.
- BBEG stays someplace where the PCs cannot kite him or engage in an artillery duel; they have to get up close in a confined space. Maybe he locks everybody in the room with him and swallows the key?
- BBEG likes extreme weather conditions (Thor's thunderstorm / hurricane? Blizzard? Heat Wave?)
- BBEG likes extreme terrain; could be Quicksand Swamp or a Cliff Face

 

[Shiroiken]

Tiamat (from HotDQ or RoT, don't remember which) or any of the Demon Lords from OOtA are excellent choices or sources of inspiration.

 

[dave2008]

Dude, I have made a bunch of them over here: 5e Epic Monster Updates

There are approximately 165 monsters currently ranging from CR20 to CR 53. Not all of them will fit the bill of what your looking for, but there are more than a few that will. However, please note these rough drafts.

One thing you can do it want a more stationary threat (which I don't recommend generally) is giving it a mechanism to protect it from range attacks: a reaction to parry range attacks, or a hide resistant (or immune) to piercing damage, or an aura the warps space around or obscures the creature from ranged attacks.

 

[Leatherhead]

I'm of the opinion that the Tarrasque isn't meant to fight the players directly, but rather be a kind of mobile natural disaster that the party has to contain before it destroys the city ala Godzilla.

That said, if you want to cover it's common weaknesses (ranged characters with enhanced mobility, which happen to be the bane of all dumb smash-happy sacks of meat) there are a few thematic options.

1. Give it a Burrow speed. This is technically just giving it flying of a different flavor though. But it does grant total cover from ranged attacks!

2. Give it a vacuum-eating "breath attack". (This should be a STR save, not DEX, which scales off the monsters CON score) Having the monster suck creatures and objects into it's maw from 300' away is a great way to really mess with standard formations and player high mobility tactics.

 

[Jaelommiss]

At challenge rating >17 each damage immunity earns a 1.25x multiplier on effective hit points. There are thirteen damage types. 1.25^13=18.190. By dividing a creature's HP by 19 (we'll round it up, because we are merciful DMs) you can give it immunity to all damage types. At CR 20 we are looking at 380 HP, which divides nicely down to 20 HP. Weaknesses include Power Word Kill and damage carried over from being hit while Polymorphed into a toad.

Honestly, I don't think we even need to add much to this to make it an interesting threat. This could very easily be placed on whatever dragon or demon or other monster you desire. The challenge comes from finding a way to effectively harm this thing.

 

[Wiseblood]

A giant ooze or colony of worms (I think the neothelid was something like that) highly resistant to mind affecting spells reactive to noxious stimuli eg. It Runs away from fire or whatever is reallly effective against it. Willing to slug it out just to devour you.

Put it in a semi-confined space with tight passages and many escape rotues since it has a less cohesive structure it could also potentially divide.

The movie Deep Rising with Treat Williams has a creature/conflict right out of D&D. I can't say it is high art but it can give Ideas.

 

[Prakriti]

Anti-magic.

Are you within a mile of the creature? Then none of your spells or magic items function.

 

[Eltab]

The monster is a swarm (like a Worm That Walks) and can split into its component parts, skitter into many miniscule holes all over everywhere, re-form in the next room, do something to toughen itself up against the present danger, and barge back into the original room for Round Two.