If you're a DM, you likely often ask yourself what kind of stat block you'd want to use for your final boss. 5th Edition is filled with demon lords and archdevils and such, but it can be a delicate thing to make these monsters feel like a real, significant threat without just utterly annihilating the party.
5th Edition uses the Challenge Rating or "CR" system to give DMs a sense of how dangerous a monster is, with formulas on how to build a challenging but surmountable encounter. In my experience, the system works... all right. Specifically, the encounter-building system from Xanathar's Guide to Everything is a lot more easy to use than the one in the DMG. But, more specifically, it's much more of an art than a science.
One thing I think is important to note is that player characters certainly do gain more damage potential as they level up, but the real power they get is more HP and more resources. At level 2, if an Ogre lands a critical hit against you, there's a good chance you're down. And at level 2, the Cleric in your party only has three spell slots, so healing you up means only two other times they can heal anyone for that day.
As you get to higher levels, though, such a thing can be trivial. A Stone Golem, which is CR 10 to an Ogre's 2, would deal about 33 damage on a critical hit, if the party is level 10, then even a Wizard with only, say, +2 to Constitution would likely have about 62 HP, meaning that that crit would only take off a little over half their health, and the Cleric is going to have fifteen spell slots, making the situation far less dire, even though the proportional threats are the same. (By the Xanathar's rules, a monster of CR equal to the party's character level is counted as an appropriate challenge for six party members).
The point is: in my experience, low-level play can often involve few combat encounters in a day - there are so few resources (including HP) for the party to expend fighting them that each combat requires them to go all out. But when you get to higher levels, any group can blast down some serious threats with ease if they have all their resources. So, even someone like the demon lord Yeenoghu can be taken down fairly easily.
But then, most boss monsters, even those slated as campaign bosses, are often in the mid 20s in terms of CR. In the Monster Manual, there's only one monster that hits CR 30, which is the Tarrasque. The Tarrasque is mainly dangerous because it puts out an enormous amount of damage and has a lot of HP, and, I'd say, its biggest advantage is its Reflective Carapace, which makes any spell attacks or line-spells automatically fail against it (sorry Warlocks). Still, while I've never run an encounter with the Tarrasque, I've heard it's not actually that scary - if you can fly above it and pelt it with arrows or area-effect spells that don't do fire damage but do require a dexterity save and aren't line spells (which is, admittedly, not a ton) you can kill it with relative impunity (at least until the DM has it pick up a massive piece of rubble and throw it at the party - which isn't a specifically delineated action they have but seems fitting).
The only other CR 30 monster printed in 5th Edition until recently was Tiamat from Rise of Tiamat. This was a kind of special boss to be fought under special circumstances, with a lot of magic items and such to allow the party a chance against her, and is just to get her shoved back into the Nine Hells.
However, Fizban's Treasury of Dragons did something I think is a clever way to approach stat blocks for gods. Gods are, you know, immortal. You might slay a demon lord in the Abyss and kill it forever (though they have ways around that,) but gods are a whole other level, and kind of untouchable. As such, I think it's clever that Fizban's gives us not stat blocks for the gods themselves, but for Aspects of them.
In interpret this as essentially being Avatars - i.e., a kind of projection of the god that is fit to be seen by and interact with mortals. And because an avatar is sort of the god producing a physical form to control in the presence of mortals, it makes sense that that physical form is not indestructible. So, honestly, I think it would be cool if we saw more Aspects/Avatars of other famous D&D gods - though Bahamut and Tiamat, being perhaps the most ubiquitous across multiple worlds, are good ones to start with.
Phew, ok, let's get on with this and look at the actual stats.
Both are similar - the same CR and built as dragons, with breath weapons and the usual damage immunities, but unlike most dragons, they do have immunity to nonmagical weapons (though their own weapon attacks are nonmagical).
Let's look at the 5-headed queen first.
First off, let's look at Tiamat's defenses.
She has an AC of 23 - which is honestly not terribly hard to hit for a high-level player. She has 574 HP, but that's a bit misleading, because she has a Mythic mode called Chromatic Wrath which resets her HP to 500 (notably, she doesn't heal to 500, so no skipping this with something like Chill Touch). She thus effectively has 1074 HP, which is significantly more than the Tarrasque (mythic monsters also give double XP, meaning she's worth 310,000 XP total, so if you're not at level 20 by the time you fight her, there's a good chance you're going to hit it).
She has immunities to all the damage types chromatic dragons can breathe, along with nonmagical weapons (which is unusual for dragons - this also means that other dragons will have a very hard time damaging her, other than gem dragons). She's got some strong saving throws - the lowest is Intelligence, with a mere +5, but the next lowest is Strength with +10, and her Con save is +19. This last is notable because without any magical way of boosting saving throw DCs, the highest a player can push their spell save DCs is 19, meaning that she will never fail on a Con save, effectively.
So, in comparison with the Tarrasque, she has a lower AC, but she effectively has more health thanks to her mythic mode, and her saving throws are generally a lot better. And with immunity to acid, cold, fire, lightning, and poison, you're going to need to dip into some less common damage types anyway (beside magical BPS).
Let's also not forget that she can fly at 120 feet per round, so unlike the Tarrasque, which you can sort of cheese, she's going to be coming for you whether you're on the ground or in an airship (she also has a swim speed - though interestingly does not have amphibious, so I think you could rule that her breath weapon can't be used without her starting to drown, technically. Though I feel like that's more of an oversight, and she should have a black dragon's amphibiousness).
Now, what about damage output?
Her multiattack involves three attacks - one bite, one claw, and one tail. These each have a +19 to hit, meaning that even a really buffed-up character like an Eldritch Knight using the Shield Spell is going to have a hard time not getting hit with that. The total average damage for all three is 86 - which is not, like, insignificant, but also not that crazy given the CR we're looking at. (Consider that a CR 16 Marilith demon deals an average of 106 damage per round if its attacks all hit - which is admittedly less likely with only a +9).
Her breath attack has a DC 27 Dex save, which is high enough that plenty of characters will simply be incapable of ever succeeding. This is also a 300-foot cone, which is enormous - and basically means you can't get beyond it - your best bet is just spreading around her so that it hits as few members of the party as possible. The damage, though, is relatively modest at 71. For comparison, a Chromatic Greatwyrm's breath does 78 damage, and an ancient red dragon's does 91. So, Tiamat's breath is bigger, but not as powerful, which is interesting.
Rather than going into all of her other abilities and legendary actions, I think I see what makes her so fearsome - while she's doing 20 less damage with a breath than an ancient red dragon, she also has nearly twice as much effective health, and her saving throw bonuses are better, and she has way more in the way of damage and condition immunities.
What we're looking at, then, is a tank. 71 versus 91 damage on a breath weapon is a big deal, but for most characters, in both cases, two of these will take you down, making the red dragon's extra damage less significant (though high-HP characters might take 3 of the former, so it's not totally insignificant).
I think the far bigger deal is that parties are going to have a much harder time burning down Tiamat's HP, and those extra rounds of survival mean more chances to recharge that breath and land those powerful blows against the party.
Let's look at Bahamut.
In terms of defense, Bahamut actually has slightly higher HP (though only a little.) He has the same AC, and swaps immunity to poison for immunity to radiant damage - which I'd argue is more common among player characters (for one thing, a Paladin's divine smites against him are useless). In terms of saving throws, Bahamut is a little more vulnerable given that he has only a +4 to Dexterity saves, but everything else is in the double digits - in fact, he has +18s to Con and Wisdom saves and +19 to Charisma saves, which again means that without some artificial boost to saving throw DCs, he's going to auto-save against any player's spells that call upon those saving throws.
Bahamut's attacks are very similar, again with +19 to hit and dealing an average of 89 damage if all three connect - so slightly more than his sister.
Bahamut has two breath weapon options. His damaging "Platinum" breath is another 300-foot cone, but this only does 66 damage and its DC is a "mere" 26. This is objectively less powerful than Tiamat's breath, though it's still a dangerous one.
However, the other breath weapon is where things get insane. Exalting Breath heals for as much as Tiamat's deals damage, in that same 300-foot cone. But furthermore, it also revives any creature that's been dead for an hour or less within it, restoring all of its hit points. And that means that this might wind up doing a lot more than 71 points of healing.
Consider, for example, that in the lore of the game, Bahamut often travels in humanoid form with seven canaries who are actually shapechanged ancient gold dragons. If he has them with him, even if those dragons go down, he can pop them right back up.
So, even if Bahamut falls behind slightly (though really only slightly) in terms of damage, I think this ability potentially makes him the far more epic challenge.
Given that he's also the lawful good, benevolent dragon god, I think that it's far less likely that the party would fight him.
Indeed, I think a good use for these stat blocks would actually be to be a manifestation of Divine Intervention when used by a Cleric. If you're a Cleric of Bahamut, and you call upon the Platinum Dragon for aid, I'd consider as a DM having the Aspect of Bahamut appear - for 1 round. Have him use his exalting breath on the party and then use his legendary actions over the course of that round, and chances are the party is going to be in much better shape.
I do think it's actually probably good design to focus on the defensive aspects of their high CR. I've heard high-level D&D described as "rocket tag," where players can "nova" for astounding amounts of damage and thus the monsters have to be able to do likewise. But I think instead making massive bosses that are resilient to such nova'ing and thus force the players to endure their attacks more might make for a more dynamic strategy.
But again, I'd have to see it in practice.
