I threw Ashardalon's lore out the window, and remade him as a destroyer of gods/worlds: he destroyed and devoured the primordial of fire in the Dawn War, and ate Mystryl some time later (long story), wiped out most of the Seldarine and Morndinsamman and I figured he needed to be pretty powerful. He's a kind of mash up of a ancient red dragon, Tiamat, and Acererak. Sooo... Is this overpowered or just right (I literally threw in everything expect the kitchen sink)

ASHARDALON
Gargantuan dragon (primordial, celestial), chaotic evil
Armor Class
23 (natural armor)
Hit Points
1000 (30d20 + 252)
Speed
60 ft., climb 40 ft., fly 120 ft.
STR
30 (+10)
DEX
11 (+0)
CON
30 (+10)
INT
27 (+8)
WIS
22 (+6)
CHA
28 (+9)
Saving Throws
Str +19, Dex +9, Con +19, Int +17, Wis +15, Cha+18
Skills
Arcana +26, Athletics +19, Deception +18, History +17,Insight +15, Intimidation +18, Perception +24, Persuasion +18, Stealth +9
Damage Immunities
fire, force; bludgeoning, piercing, and slashing from nonmagical attacks
Condition Immunities
blinded, charmed, exhaustion, grappled
Senses
blindsight 120 ft., darkvision 240 ft., truesight 120 ft., passive Perception 34
Languages
All
Challenge
30 (155,000 XP)
Aura of Fire and Magic.
Each creature of Ashardalon's choice that starts its turn within 60 feet of him takes 20 fire damage and 20 force damage.
Legendary Resistance (5/Day).
If Ashardalon fails a saving throw, he can choose to succeed instead.
Magic Resistance.
Ashardalon has advantage on saving throws against spells and other magical effects.
Magic Weapons.
Ashardalon's weapon attacks are magical.
Spellcasting.
Ashardalon is a 20th-level spellcaster. His spellcasting ability is Charisma (spell save DC 26, +18 to hitwith spell attacks). Ashardalon knows the following sorcerer spells:
Cantrips (at will):
chill touch, fire bolt, prestidigitation
1st level (at will):
detect magic, false life, fog cloud, magic missile, shield
2nd level (at will):
darkness, detect thoughts, gust of wind, mirrorimage, misty step
3rd level (at will):
animate dead, counterspell, dispel magic, fireball, vampiric touch
4th level (3 slots):
fire shield, greater invisibility, wall of fire
5th level (3 slots):
cloudkill, cone of cold, wall of force
6th level (3 slots):
chain lightning, circle of death, create undead, disintegrate, globe of invulnerability
7th level (3 slots):
delayed blast fireball, finger of death, forcecage, plane shift
8th level (2 slots):
dominate monster, incendiary cloud, maze
9th level (2 slots):
imprisonment, meteor swarm, power word kill, time stop
ACTIONS
Multiattack.
Ashardalon can use his Frightful Presence. He then makes three attacks: one with his bite and two with his claws.
Bite.
Melee Weapon Attack:
+19 to hit, reach 15 ft., one target.
Hit:
32 (4d10 + 10) piercing damage plus 14 (4d6) fire damage.
Claw.
Melee Weapon Attack:
+19 to hit, reach 10 ft., one target.
Hit:
24 (4d6 + 10) slashing damage.
Tail.
Melee Weapon Attack:
+19 to hit, reach 20 ft., one target.
Hit:
28 (4d8 + 10) bludgeoning damage.
Frightful Presence.
Each creature of Ashardalon's choice that is within 120 feet of Ashardalon and aware of him must succeed on a DC 26 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Ashardalon's Frightful Presence for the next 24 hours.
Fire Breath (Recharge 5-6).
The dragon exhales fire in a 100-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 104 (30d6) fire damage on a failed save, or half as much damage on a successful one.
LEGENDARY ACTIONS
Ashardalon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Ashardalon regains spent legendary actions at the start of its turn.
At-Will Spell.
Ashardalon casts one of his at-will spells.
Detect.
Ashardalon makes a Wisdom (Perception) check.
Tail Attack.
Ashardalon makes a tail attack.
Wing Attack (Costs 2 Actions).
Ashardalon beats his wings .Each creature within 15 ft. of Ashardalon must succeed on a DC 27 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. Ashardalon can then fly up to half its flying speed.

Those DCs are really high. Some characters in legitimate party compositions would just never be able to save against that Frightful Presence, for example. I'm pretty sure DCs are intended to be bound in the same way AC is. The breath  weapon damage and DC seems like a lot too.

At this level, it's just really hard to say because the system constraints are a lot looser and too much other stuff matters. What's the battleground? How much prep does the party get? How optimized are their characters? What's their gear? What are the party strengths and weaknesses based on their class/subclass composition? I haven't found a great way to balance high level creatures beyond keeping track of what works and what doesn't over many encounters and basically customizing monsters to that particular group of adventurers.

Quote from scatterbraind >>

Those DCs are really high. Some characters in legitimate party compositions would just never be able to save against that Frightful Presence, for example. I'm pretty sure DCs are intended to be bound in the same way AC is. The breath  weapon damage and DC seems like a lot too.

At this level, it's just really hard to say because the system constraints are a lot looser and too much other stuff matters. What's the battleground? How much prep does the party get? How optimized are their characters? What's their gear? What are the party strengths and weaknesses based on their class/subclass composition? I haven't found a great way to balance high level creatures beyond keeping track of what works and what doesn't over many encounters and basically customizing monsters to that particular group of adventurers.

 The battleground’s in the middle of a volcano (lots of lava!) Characters are really powerful thanks to some artifacts and the fact they are the Chosen of Correllon, Clangeddin, and Erevan Iliesere respectively. They have a lot of ways to negate damage, that’s why the DCs and damage are so high. The party have a lot of prep (they went to Candlekeep just to research him). They are also immune to being frightened (thanks, gods), and the Frightful Presence is just to get rid of their allies. I don’t need to use this guy for a few months, so he’ll probably undergo a few more changes. This is just to get an idea of his strength, really.  

yeah the DCs are definitely excessive. even with a +10 modifier there's still a high chance to fail, and without at least a +8 modifier, chance of success is a measly 5% (or less if you don't allow nat-20s = instant success).

Overall this looks really overblown, honestly I think you'd be better off just giving an ancient red dragon spellcasting and calling it a day.

Quote from PelinalLovesick >>

yeah the DCs are definitely excessive. even with a +10 modifier there's still a high chance to fail, and without at least a +8 modifier, chance of success is a measly 5% (or less if you don't allow nat-20s = instant success).

Overall this looks really overblown, honestly I think you'd be better off just giving an ancient red dragon spellcasting and calling it a day.

 I was gonna do that, but since I’ve built him up to be even more powerful than most gods (forget demon and devil lords, he’d slaughter them in 30 seconds), I felt his stats should reflect that. I’m probably gonna give the party potions of invulnerability, so half his damage is negated.

The DC actually matches the dragon stat, with the usual 8+prof+stat mod. And if you aim for godhood power level, I think those DC are fine, comparing to Tiamat avatar, for example, the DC are the same.

Depends on the party. If there's a paladin in the party, or if they have access to any number of spells that make PCs immune to being frightened, it may be too easy. A smart party would also have something to make them resistant to fire damage, and use ranged weapons + sapping sting (which is basically a dragon-killer). Also, this guy isn't immune to being stunned, restrained, or paralyzed, so a monk with stunning strike or a hold monster spell could lock it down, or at least have it burn through legendary resistances. I would consider making it immune to those conditions to make sure that the action economy doesn't get away from the monster.

If this guy is a god, then limited magic immunity may also be a worthwhile ability to throw in.

Quote from Heartofjuyomk2 >>

Depends on the party. If there's a paladin in the party, or if they have access to any number of spells that make PCs immune to being frightened, it may be too easy. A smart party would also have something to make them resistant to fire damage, and use ranged weapons + sapping sting (which is basically a dragon-killer). Also, this guy isn't immune to being stunned, restrained, or paralyzed, so a monk with stunning strike or a hold monster spell could lock it down, or at least have it burn through legendary resistances. I would consider making it immune to those conditions to make sure that the action economy doesn't get away from the monster.

If this guy is a god, then limited magic immunity may also be a worthwhile ability to throw in.

 Yeah, I think I’m going to throw in immunity to spells of 6th and lower, as well as the conditions you mentioned, and allow his fire stuff to overcome resistance. Worst comes to worst, I’ll let him do the fire breath as a legendary action, like Tiamat.

looking at this why was this thread even made the title is like "is the most powerful ranking in the game too weak for this being i have made" (sorry if you played mostly early additions where cr 30 was not the highest 50 something was)

He's got a lot of hit points.  His breath weapon does a lot of damage.  There is no mention of a save or any other way to avoid the damage of his aura aside from immunities or resistances.

He can fly around and roast the party and all their minions from 100 feet away, well inside the range of his Frightful Presence.  Looks like three breaths will take out the toughest Barbarian with some good rolls for the dragon and poor rolls for the Barbarian.  He has a fair chance of doing so in 3 rounds.

I see no mention of Lair Actions or Minions.

He's oddly lacking in immunity to elemental damage, acid, poison, necrotic, radiant, psychic, or even lightning.  He's not likely to make any dex saves.  He can be poisoned, diseased, or cursed.

Looks like a fight would be over quickly.  If the players can't force him to land, he gets to roast them with his breath.  A nice Meteor Swarm to soften people up and a few breaths later it's all over.  Unless the party can avoid the fire damage, one of the most common damage types around.  He's got a few nice spells, but he has to get in range to use them.  Counterspell is nice.  Dispel Magic could be handy.  The rest are pretty questionable.  Things like Concentration, or short ranges, small amounts of damage and that mostly Fire...  His damage output on the ground isn't impressive and he's got no way to heal.

Lair actions

Volcanic burst

DEX save or 10d6 fire on a success half also they have to make a DC 27 STR save or be knocked prone