Looking at the Greatwyrms

 We just looked at the CR 30 monsters from Fizban's Treasury of Dragons, the Aspects of the two major dragon gods, Bahamut and Tiamat. These are the only stat blocks aside from the adventure NPC stats for Tiamat (not just her aspect) in Rise of Tiamat to match the Tarrasque at this highest of challenge ratings. I looked at how their damage output is not absurdly high (though they're no slouches) and how, instead, their major challenge comes from likely being able to endure several rounds of punishment from a high-level party.

If high-level D&D is rocket tag, the Aspects have a pretty sophisticated missile-defense system, so the strategy of just taking out the boss as quickly as possible won't necessarily work.

These aren't the only super-high CR monsters out of Fizban's Treasury of Dragons. The other is the Greatwyrm. Fizban's revealed that dragons on each world of the prime material plane are sort of one iteration or incarnation of a fractured draconic soul. Dragons who can bind to their counterparts across the myriad worlds (whether through cooperation or conquest) can become super-powerful, transforming into something called a Greatwyrm, which (in theory) is more powerful than even an ancient dragon. (Yes, this is more or less the plot of the 2001 Jet Li film The One.) 

The book provides us with stat blocks for three broad archetypes - the Metallic, Chromatic, and Gem Greatwyrms. Each essentially has a couple of variables when it comes to damage types, but beyond that the stats are identical to their branch of dragonkind. Metallic Greatwyrms are CR 28. Chromatic ones are 27, and Gem ones are 26.

This does have a few interesting consequences. For example, a Blue Greatwyrm's breath will be a cone rather than its usual line, and likewise a Green Greatwyrm's breath calls for a Dexterity saving throw rather than a Constitution one. I think DMs would be justified in altering these, though I'd personally, at least in the case of the Blue one, treat it as a massive cone-shaped barrage of lightning, or forked lightning.

I do find it interesting that the three are different CRs - as I said when looking at the two Aspects, I think Bahamut has the potential to be a far tougher fight than Tiamat given his ability to pump out insane amounts of healing (though only if he has allies - otherwise Tiamat has the clear edge). Metallic Dragons tend toward goodness, though Fizban's made it clear that as beings of the Prime Material Plane (indeed, the ones most tied to that plane, even more than us humanoids) Dragons are beings of free will, and there are suggested personality traits that can make for evil-aligned Gold dragons or good-aligned Black dragons. Thus, there's nothing preventing the party from having a Metallic Greatwyrm as their big bad.

Still, one would hope that CR 27 or 26 would make for satisfyingly challenging final bosses. Let's take a look at all of them.

Metallic Greatwyrms:

First off, I want to look at defenses. Metallics get a base HP of 565 and are, like the Aspects, Mythic monsters that reset to 450 HP when you get to "phase 2," which means that they have effectively 1015 HP - honestly not much behind the Aspects. And, indeed, they don't have a single saving throw bonus that's below double digits - ranging from a +10 to Strength (no proficiency) to +18 to Charisma saves. Their AC is 22, though, which for a high-level threat is not terribly hard to hit.

They do have fewer condition immunities than the Aspects, though they still have 4 legendary resistances (compared to the Aspects' 5) which also reset when they go Mythic.

I think by far the biggest reduction in defense is the lack of so many damage immunities. Because Bahamut and Tiamat are immune to every kind of damage their type of dragon can have immunity to (with Bahamut getting Radiant instead of Poison) that seriously reduces the sort of harm that any spellcasters can bring to them. Greatwyrms only have their color/metal/gem's specific damage immunity, so even nonmagical weapons can hurt them. A Silver Greatwyrm will be subject to your Chain Lightnings, your Firestorms, or even your Summon Elemental's Air Spirit's slam attacks.

Movement-wise, they all inherit the movement speeds of the rest of their dragon type - so even though a Silver dragon doesn't have a swim or burrow speed, they get this (which might just be because they decided to use a single stat block for all metallic dragons).

On the offense, their multiattack is limited to just two attacks (the Aspects get Bite, Claw, and Tail, while the Greatwyrms get just Bite and Claw, saving their tail attack for either legendary actions or opportunity attacks). Thus, when they don't have their breath weapon, they're putting out about 53 damage per round, with a +18 to hit. The hit bonus is only slightly behind the Aspects, and only very rarely will miss, but the lack of that third attack does significantly lower their damage.

Metallics get two breath attacks, as is typical. The Elemental Breath is your standard one, and requires a DC 25 Dex save against 84 (on average) damage. 25 is... a little more doable, but still means an auto-fail for anyone without proficiency in Dexterity that doesn't have it maxed out (and if you don't have proficiency, you probably don't have it maxed out). Indeed, that 84 is actually more damage than the Aspects do.

The other breath is Sapping Breath, which requires a DC 25 Con save or the creature goes unconscious for 1 minute, but even on a success, the creature gets disadvantage on attack rolls and saving throws until the end of the dragon's next turn. So, that's a pretty big deal, and can put several members of the party down. Again, without buffs, if you aren't a class with Constitution saving throw proficiency or maxed out Con, you're an auto-fail, which is pretty nuts. Of course, things like Bless or a Paladin's Aura of Protection can significantly help here.

I have to imagine than any Metallic Dragon is going to start things off with their Sapping Breath, which could potentially knock out most of if not the entire party on the first round. Even a Barbarian with +7 to Constitution, and thus a +13 to Con saves, is more likely than not to fail that. Unconscious creatures do get to repeat the save at the end of their turns, but again, it's a hard one to hit - and it doesn't break on damage, so if the Greatwyrm wants to kill the party, they're going to have easy pickings (and auto-crits on their attacks).

Thus, this feels like one of those creatures you've got to have a really good plan for - and ensure you approach it from multiple angles so you don't all get hit by that breath from the start.

Chromatic Greatwyrms:

Despite having a higher Con modifier than Metallics, Chromatics have less health because they have fewer hit dice. Their AC and speeds are the same, along with condition and damage immunities. However, they do have a weakpoint which is Intelligence saving throws, to which they get a mere +5 (the rest are all at least +10). They start with 533 HP and reset to 425 when they go Mythic, putting them at less than 1000 HP overall, though still more than the Tarrasque.

The Chromatic's attacks are familiar, and do identical damage to the Metallic's with the same bonus to hit.

They only have their one Breath Weapon, which is a DC 26 Dexterity save and deals 78 (average) damage in a 300 foot cone. This is less damage (though not by a ton) though I will note that unless you have some means to push a saving throw or your Dexterity higher than normally possible, no one without proficiency in Dexterity saving throws has a chance of succeeding on this.

Still, compared to the Metallic Greatwyrm, I think we're looking at much more of a straight damage-dealer. Nothing here really debilitates the party without knocking them out from damage. They are of course fast and have a lot of HP, so I think the usual strong dragon strategy of flying away from the party and only turning around when you get your breath weapon back (and then using it from as far away as possible) is probably the best option, but I could easily imagine a clever party finding a way to lock one of these down and deal with it swiftly. There's nothing here as devastating as the Sapping Breath.

Gem Greatwyrms:

One of the interesting things here is that their damage immunity and breath weapon are going to be a more unusual damage type. An Amethyst Greatwyrm is going to infuriate the party's Warlock, and a Crystal Greatwyrm is going to make a Paladin far less effective.

Defensively, we're looking at slightly lower AC, with just 21. Their HP is an average of 507, with a reset to 400 at the mythic transition. Saving Throws are also notably a little less intense - no +18s or +19s, and a few with a "mere" +10 or +9 - so Dex, Strength, and Intelligence saving throws are - by the other Greatwyrms' standards - pretty low.

Notably, though, because Crystal Dragons hover, they're immune to the prone condition - which might not come up a lot, but it does mean that attempts to ground the Greatwyrm could be a lot tougher, which genuinely makes for a more difficult fight, potentially.

On the offense, their attacks have a +17 to hit - again, this will probably hit almost all the time, though it's getting a bit more possible for it to miss. The total damage of its bite and claw attacks come to 54 - oddly very slightly higher than the others, though I think the lower hit chance would probably eliminate that bonus.

Their breath weapon requires a DC 25 Dexterity saving throw and does 71 damage - but it also knocks foes who fail it prone, which, if nothing else, will likely slow the party's ability to reposition themselves (and opens them up to more likely crits from legendary claw attacks).

Interestingly, Gem Greatwyrms don't get Mythic Actions to use as Legendary Actions, but instead get a new option they can use once per short rest - which is Mass Telekinesis. This requires basically the whole party to make a DC 26 Strength saving throw or be restrained, lifted up by the telekinesis. Once lifted, the dragon can move the creatures around and automatically deal 45 force damage to them at the end of each turn. The creature can repeat their save at the end of each turn, but the greatwyrm also gets to move them up to 60 feet per round, so there might be some fall damage there too.

Also, Gem Greatwyrms get psionic spellcasting with one cast per day of each dispel magic, forcecage, planeshift, reverse gravity, and time stop.

So, even though the Gem Greatwyrm is the "weakest" in terms of its defensive abilities and damage output, I think that the spells it gets and in particular its Mass Telekinesis give it the potential to be a really profound danger to the party.

    All in all, I find it interesting that the Chromatic Greatwyrms, which seem the most likely to wind up as big bads of a campaign, actually seem the least threatening to a party. The CR makes sense as a product of their defenses and damage output, but they seem the type of monster that is very "solvable" by a high-level party with lots of spells and abilities.

    The Gem Greatwyrm looked a little flimsy at first, but I think it's got some strong tools to terrify your players. But the Metallic Greatwyrm's Sapping Breath is, I think, the ability that makes them utterly insane.