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
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
Base Attack/Grapple: +22/+43
Vulnerabilities (x1.5 damage): [Cold] (Stage 1)
Resistances (x0.5 damage) : [Fire] (Stage 2)
Immunities : [Fire] (Stage 3)
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.
One bite, two claws, one tail.
Reach 10 ft., 1d12+11 [Piercing] damage.
Reach 5 ft., 1d8+11 [Slashing] damage.
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.
Feats and Paragon Traits
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.
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.
A dragon has four natural attacks, which can all be done simultaneously with a -2 on the attack roll after the first 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.
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?