Check out the Astral Deadnought from Mordenkainen's Tome

[jimmifett]

This looks... familiar... Dreaming Dark from Eberron...?

[Ralif Redhammer]

Gloriously dangerous. The Demiplanar Donjon means that someone that gets swallowed is pretty much on their own to make their death saves, or not.

Though my favorite illustration will be the 1e Manual of the Planes cover, this one does it justice.

[MarkB]

I haven't looked at many really high-level monsters, but 297 hp kind-of feels low. You could whittle that down even with just up-levelled fireballs and lightning bolts pretty quickly, and in any environment large enough to accommodate this thing, its anti-magic cone isn't going to be able to keep high-level spellcasters locked down very easily.

I definitely want to see an Astral Deadnought, though - they sound awesome.

[vincegetorix]

ah, my players call BS when I throw more than 4 goblins at them. Cant wait to use this one, they'll know the true meaning of fear

[CapnZapp]

Let's see what @dave2008 's version looks like.

[Flexor the Mighty!]

The HP are weak, just like pretty much every high level 5e monster.

[TerraDave]

I haven't looked at many really high-level monsters, but 297 hp kind-of feels low. You ...

Its low. The DMG recommends 400-445 for a cr 21 monster. And an AC of 19 vs. the 20 it has.

[Saelorn]

Is that supposed to be scary? Its big three-legendary-action special ability does 15 damage, with a save for half.

A level 20 barbarian could take this thing in a fight. It just doesn't have the numbers. I guess the anti-magic cone is supposed to be scary, but that only works in one direction at a time, and there's nothing stopping you from just running in a circle around it.

[MonsterEnvy]

Its low. The DMG recommends 400-445 for a cr 21 monster. And an AC of 19 vs. the 20 it has.

You are forgetting about damage resist and offensive CR.

Is that supposed to be scary? It's big three-legendary-action special ability does 15 damage, with a save for half.

A level 20 barbarian could take this thing in a fight. It just doesn't have the numbers. I guess the anti-magic cone is supposed to be scary, but that only works in one direction at a time, and there's nothing stopping you from just running in a circle around it.

It's harder to run around the Antimagic cone when you are far away. But yeah it's damage seems a bit low. And while it does have quite a bit of HP it's actually slightly lower then the One CR Below it Pit Fiend.

I haven't looked at many really high-level monsters, but 297 hp kind-of feels low. You could whittle that down even with just up-levelled fireballs and lightning bolts pretty quickly, and in any environment large enough to accommodate this thing, its anti-magic cone isn't going to be able to keep high-level spellcasters locked down very easily.

I definitely want to see an Astral Deadnought, though - they sound awesome.

How you you going to toss the Fireballs and Lightning Bolts at it when you are in the Anti Magic Cone.

[Voort]

An underwhelming opponent in a straight-up brawl, sure. But against PCs using an Astral Projection spell, that Sever Silver Cord ability is a 5% chance of instant death three to six times per round. Would make a nasty ambush predator...

[Brightmane]

Also consider that it can potentially remove a problem opponent each round with Donjon Visit, effectively locking out said 20th level barbarian while it kills a softer target. It is also faster than most players on the Astral, allowing it to do hit and run attacks. If the party chases it, they risk spreading themselves out.

[MonsterEnvy]

Also consider that it can potentially remove a problem opponent each round with Donjon Visit, effectively locking out said 20th level barbarian while it kills a softer target. It is also faster than most players on the Astral, allowing it to do hit and run attacks. If the party chases it, they risk spreading themselves out.

According to Xanathar's this thing should be over powered by the characters if they are level 20. Level 19 for 4 characters or level 17 for 5 players are the levels this thing should be more equal to them on.

[Sword of Spirit]

It looks like they are trying to make an opponent that is more interesting in combat, rather than just a big bag of hit points (like people keep asking for). To do so, the raw numbers that determine CR will have to be less than similar creatures. That said, I do feel like the damage resistance is useless at this level, so I hope that's just a ribbon not considered part of CR calculation.

With up to 6 attacks per round, that chance of a critical that instantly slays a character is not insignificant, and it could make for a tough fight if the antimagic field and legendary donjon action are wielded by the DM with skill.

[Kobold Avenger]

A level 20 barbarian could take this thing in a fight. It just doesn't have the numbers. I guess the anti-magic cone is supposed to be scary, but that only works in one direction at a time, and there's nothing stopping you from just running in a circle around it.

No need to run, since it's encountered only in the Astral Plane where gravity is subjective and there's very little in the way of a "ground" anywhere, you can fly around it without magic.

[MonsterEnvy]

That said, I do feel like the damage resistance is useless at this level, so I hope that's just a ribbon not considered part of CR calculation.

Actually it's damage resistance is quite useful because it has that antimagic cone.

[werecorpse]

Hmm ok...

...but seriously, who uses astral projection anyway.

[Prakriti]

Is that supposed to be scary? It's big three-legendary-action special ability does 15 damage, with a save for half.

A level 20 barbarian could take this thing in a fight. It just doesn't have the numbers. I guess the anti-magic cone is supposed to be scary, but that only works in one direction at a time, and there's nothing stopping you from just running in a circle around it.

A level 20 barbarian has to succeed on a DC 19 Charisma saving throw every round or lose its turn. That's one dead barbarian, if you ask me.

[dave2008]

Its low. The DMG recommends 400-445 for a cr 21 monster. And an AC of 19 vs. the 20 it has.

FYI, the DMG list effective HP. Per the guidelines you would have to multiply the 297 x1.25 (resistances) which gives an effective HP of 371, + 30 for each use of Legendary resistance, so 461 effective HP and an effective AC of 22 (maybe higher w/ the antimagic cone):

By my quick calculation you get:

Defensive CR: 23 (see above)
Attack CR: DPR = 131 = 20 + 3 for attack bonus (16-10)/2 = CR 23

So that should be total CR 23. No I am guessing they didn't use the resistance multiplier (though the said they assume no magic weapons, this implies otherwise). So you get:

Defensive CR: 20
Attack CR: 23

Total CR 21.5, rounded down to 21. Seems by the book to me.

[Prakriti]

Never mind.

[dave2008]

Is that supposed to be scary? It's big three-legendary-action special ability does 15 damage, with a save for half.

It is a bit weak for me too; however it is pumping out a 131 DPR. The legendary action is 15 to all opponents (still not enough IMO), but I used three claws at 19 to figure the DPR.