What is the Challenge Rating of this custom “Volcanic Dragon”?

Question

I came across the sheet of a super awesome Volcano Dragon, custom made for D&D 5e by someone much more experienced than me.

I wanted to use that sheet, but we play 3.5e instead of 5e, so I had to make some tweaks to the sheet, as seen here:

Vulcano Dragon (End Game content!) Reference document: https://drive.google.com/file/d/0B2RlGhhWYcFtRjNsR2xOTnRUTFU/view

Size: Gargantuan
Speed: 40 ft.; Fly 100 ft. (clumsy)
HP: 22d12+132 per stage. (200 per stage)
AC: 40 at Stage 1, 38 at Stage 2, 35 at Stage 3
Str: 32
Dex: 10
Con: 22
Int: 15
Wis: 16
Cha: 18

Base Attack/Grapple: +22/+43
Attack: +31/+26/+21/+16/+11
Fort: +19
Ref: +13
Will: +17

Vulnerabilities (x1.5 damage): [Cold] (Stage 1)
Resistances (x0.5 damage) : [Fire] (Stage 2)
Immunities : [Fire] (Stage 3)

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Hardening Skin (Stage 1):
After taking [Cold] damage, speed reduced by 20 ft. and can't fly.

Blazing Form (Stage 3):
Touching/Attacking the dragon while standing within 5 ft. causes you to take 1d10 [Fire] damage.

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Actions

Full attack:
One bite, two claws, one tail.

Bite:
Reach 10 ft., 1d12+11 [Piercing] damage.

Claw:
Reach 5 ft., 1d8+11 [Slashing] damage.

Tail:
Reach 15 ft., 1d10+11 [Bludgeoning] damage.

Lava Bomb (Stage 1 and 2) (Recharge 5-6):
60 ft. range, 10 ft. diameter impact. 7d6 [Fire] damage. Save DC 24 (Adjust accordingly), Reflex for half damage. If failed, 3d6 extra [Fire] damage on the end of the next turn.

Earthquake Stomp (Stage 1):
10 ft. range from self, 5d6 [Thunder] damage. Save DC 22 (Adjust accordingly), Fort for half damage. If failed, pushed away 10 ft. from dragon.

Shed Embers (Stage 2):
While flying, centered on self, 50 ft. downward, 10 ft. diameter impact. 6d6 [Fire] damage. Save DC 23 (Adjust accordingly), Reflex for half damage. Area slightly obscured for one round.

Lava Vomit (Stage 3) (Recharge 5-6):
15 ft. cube in front of the dragon, 8d6 [Fire] damage. Save DC 25 (Adjust accordingly), Reflex for half damage. Area remains filled with lava for one round. Entering the lava does 4d6 [Fire] damage.

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Feats and Paragon Traits

Paragon Fortitude:
Multiple health-pools, each tracked seperately. All damage and healing can only be applied to one pool. If a pool is reduced to zero, all ongoing effects on the creature end. Once a pool is reduced to zero, it can't regenerate until after a long rest. If all pools are reduced to zero, the creature dies.

Paragon Fury:
The dragon may take an additional turn each round for each of it's hitpoint-pools that have been reduced to zero, regardless of initiative order. For the first turn initiative is rolled normally, though it gains advantages on the roll.

Multi-attack:
A dragon has four natural attacks, which can all be done simultaneously with a -2 on the attack roll after the first attack.

Power Attack:
Subtract melee Attack Bonuses and add them to the damage. (Limited to the BAB!)

Improved Bull Rush:
Bull Rush provokes no AoO and the dragon gains an additional +4 bonus on the Strenght-checks regarding bull-rushes.

Awesome Blow:
Subtract 4 from melee Attack Bonuses and knock creatures back 10 ft. in the direction of the dragon's choice and fall prone. If an obstacle prevents the creature from completing the movement, both the creature and the object take 1d6 damage. Reflex negates, DC = damage dealt. ONLY WORKS IF THE CREATURE IS CORPEREAL AND SMALLER!

I especially have a hard time estimating what would be appropriate save DC's, or damage dice, since I personally have never played a high-level character or DM'd a high-level party, hence my question.

Would this be a difficult encounter against a party of 4 level 16 characters? If not, how can I adjust it to be an appropriate CR?

Answer 1

Yes it is more or less appropriate challenge, though it should be something like "boss"/"mini-boss".

As TheDarkWanderer pointed out Challenge Rating and average party level shouldn't necessarily match, so your party should probably be ok with a creature like that. After all CR system doesn't describe challenge really accurately, as it depends on multitude of different things like pre-scouting and builds.
You may still want to adjust this dragon for it to suit your needs more, but here is estimated 3.5e statblock based on information you've given:

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Size/Type: Gargantuan Dragon (Fire)

Hit Dice: 22d12 +132 (275 hp) x3

Initiative: +0

Speed: 40 ft. (8 squares), fly 100 (clumsy)

Armor Class: 40 (-4 size, +44 natural [decreses slightly in later forms]), touch 6, flat-footed 40

Base Attack/Grapple: +22/+45

Attack: Byte +29 melee (4d6+11)

Full Attack: Byte +29 melee (4d6+11) and 2 claws +27 melee (2d8+5) and 2 wings +27 melee (2d6+5) and tail slap +27 melee (2d8+16)

Space/Reach: 20 ft./15 ft. (20 ft. with byte and tail)

Special Attacks: Crush, Tail Sweep, Lava Bomb, Earthquake Stomp, Shed Embers, Lava Vomit

Special Qualities: Darkvision 60 ft., Low-light vision, Dragon traits, Vulnerability to Cold, Immunity to Fire, DR 10/magic, SR 26, Paragon Fortitude, Paragon Fury, Blazing Form, Hardening Skin

Saves: Fort +19, Ref +13, Will +16

Abilities: Str 32, Dex 10, Con 22, Int 15, Wis 16, Cha 18

Skills: whatever

Feats: Multiattack, Power Attack, Improved Bull Rush, Awesome Blow (and 4 more feats of DM’s choice)

Challenge Rating: ~18

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Crush (Ex) (4d6+16) DC 32; Tail Sweep (Ex) (2d6+16) DC 32
Read this about dragon specials. Note, while it is generally suggested to base these attacks on Con, I personally suggest to base them (as well as Earthquake Stomp) on Str instead, as they are basically area melee attacks.

Earthquake Stomp (Ex) (~2d6+16) DC 32
It is hard to convert this into 3.5 safely, since 3.5 is much less cartoon-styled, so just stomp would hardly be capable of causing Sonic (or what alternative is appropriate for Thunder?) damage. You may want to completely rework it.

Blazing Form (Ex) (~ 3d6)
Lava Bomb (Su) (~ 7d10) DC 27; Shed Embers (Su) (~ 9d10) DC 27; Lava Vomit (Su) (~ 12d10) DC 27
These are based on appropriate Breath Weapons of similar dragons.

I’ve added Fire subtype here, making Vulnerability to Cold and Immunity to Fire constant. You may of course reverse this, but it is how 3.5 generally deals with such cases, I believe.
Damage and Reach are now set as appropriate for gargantuan dragons. I added longer reach for the tail as it seems to have longest reach in the source. Gargantuan dragons usually also have Wing attacks (as well as Crush and Tail Sweep specs). Feel free to remove them if needed.

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Bear in mind that this creature will became MUCH stronger when it will lose two of it’s pools. So it isn’t CR 18 from the start, but maybe somewhere close at it’s third pool.

Answer 2

There will be a lot of work ahead of you to try and adapt that particular dragon to 3.5/Pathfinder rules. Some things simply don't exist, such as Paragon Fortitude, and will be very difficult to try to mimic in the confines of published material. Rather than starting from stage 0 and trying to get to the final stage, check out some published dragons, which will offer a true challenge to multiple higher level characters.

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If you want a volcanic "feel" for a dragon, you could use the Pyroclastic Dragon¹ (Draconomicon) or the Magma Dragon (Pathfinder). Both of those dragons are published and utilize attacks designed around the forces of volcanic elements.

¹Great Wyrm Pyroclastic Dragons have a DC 40 (Base) Fortitude Save or be disintegrated breath weapon. Hence the epic comments from KRyan and Sandwich.

Answer 3

What is the Challenge Rating of this custom “Volcanic Dragon”?

"By-the-book", you would use this. Most would be quick to point out, it isn't always a good gauge. Also, if you do a few walk-throughs with existing monster entries, you'll find it was likely very subjective when applied to those entries. That or wasn't formalized until after the entries. Some do, however, line right up.

Having said that, a quick wag is

• +1 per 2HD of Dragon = 11
• size gargantuan = +1 for large or larger, 12, for gargantuan, 14
• "By-the-book", you add 1 to CR for a special ability that "improve combat effectiveness in a minor way" and 2 for "special attacks or qualities that significantly improve combat effectiveness" - the creature listing has at least 5 special abilities (feats taken at 1st, 3rd and every 3 levels thereafter don't count towards this). If they "improve", then +5 for 19, if they "significantly improve" then +10, 24.

This could clearly go higher or lower if you analyze each ability and determine they contribute more or less.

You can adjust it by adding or taking away from size, special abilities or hit dice.