Ancient Black Dragon

So, when running the pathfinder campaign, the players found a map of feyblight, and saw a note left on the map during its creation that I had not erased completely.  It was simply a 20, nothing else.  I had used the method of populating the map by dropping dice on it with the CR of the monster/encounters in what locations.  It's a fun way to do maps on the quick, and can help you break through writer's block as you figure out how various ecosystems and political boundaries when you have to explain why there is a camp of vikings hanging out next door to an ancient black dragon.

So anyways, when making the map I determined that the 20 was an ancient black dragon, and fleshed it out to see just how badass they could get.  Naturally, when the players spotted the '20' on the map, they made a b-line there. They were all level 10+ and felt like legit badasses, and they were, but this was the big level boss for when they finished their current questline which I was hoping they'd be at least level 15 for.  It took me three weeks to explain to them how it would be a suicide if they followed through with their plan, they were very determined and only on the day when they were going to finally get there did they fall back.  I actually had to place a bet about how many rounds it would take the dragon to take the party out.

Looking over the notes, I'm left wondering if level 20 encounters are actually any fun at all?  There's a LOT to be keeping track of.  Usually my campaigns wind down around 10-14 ish.

Anyways, here are my notes for Darakna:

The highlighted portions are if it is aware of the approach of the PCs and has time to buff before it strikes.  The bold portions are the spells it's using for said buffs.

The PCs were approaching during winter, and the swamp was mostly iced over, so the dragon's strategy was going to be:

1. Prep: Cast buff spells.
2. Prep area: Cast dimensional anchor on the area. 
3. Surprise attack: Use its breath weapon to dissolve the ice they were walking over so the party would fall into the deep portion of the swamp, getting stuck in the mud with their heads under water.
4. Cast transmute mud to rock to trap party members under water and make things more manageable.
5. Cast darkness 100' and use dragon senses for targeting spells.
6. Destroy the party.

The black dragon Darakna(hobgoblin) AKA Klepisis the eternal gloom(elven)  AKA  Dynythic  the reaper of tears(frost giant), The black rose(dragon), Kastorkir (druid)

Nearby Offspring: Norakna--Dracolisk.

Onrassk -Juvinile black dragon

Korrasin - hatchling in lair

Spells:

Caster level 12 save DC 14+spell level (16+spell level 8 rounds to fully buff)

per day(after buff)

3(3) Level 6 - flesh to stone, mislead(EA)

4(4) level 5- baleful polymorph(usually into beavers, the dragon likes beavers, Fort, will), trans mud to rock, feeblemind(EA)

6(6) level 4- dimension door, dimensional anchor(ray, touch, no save), resiliant sphere(EA), detect scrying(always cast)

7(6) level 3- dispel magic, herosim(+2 attack, save, skills), ray of exhaustion(ray touch, fort for fatigue),   sepia snake sigil(reflex or put in stasis)

7(4) level 2- blur(20% miss), invisibility, eagle splendor(+4 chr), see inv, detect thoughts

7(14RoW)((4)(10RoW) ) level 1- mage armour(+4 touch ac), shield(+4 touch ac, block MM), magic missile(5d4+4), ray enfeeblemen(ray, touch, 1d6+4 str damage, fort half), exp retreat(+30 movement)

at will level 0- dancing lights, det. magic, light, mage hand, mending, message, prest. r. magic, create water.

total buffs: +30 move, +8 touch ac, +2 att, +2 save, +2 skills, see inv, +4 chr.

Feats:

• imp natural weapon bite (bite goes from 2d8 to 4d6)
• wingover(180 turn no move penalty)
• fly by attack (move-standard action-move doesn’t provoke from target)
• greater vital strike (single attack that does 4x dice damage)
• improved initiative (+4 init)Xextra sorcerer spell expanded arcana
• improved vital strike (precursor to gvs)
• multi attack (reduce penalties for full attack)
• power attackXditch for hover
• skill focus stealth (+6 stealth)Xexpanded arcanal
• vital strike (precursor to ivs)
• weapon focus bite Might ditch for improved critical for that 19-20 threat range on the bite attack.

XP 76,800

CE Huge dragon (water)

Init +4; Senses dragon senses; Perception +30

Aura frightful presence (300 ft., DC 25 (27 buff))

DEFENSE

AC 38, touch 8, flat-footed 38 (+30 natural, –2 size)

Buff AC(46, touch 16, flat-footed 46 (+30 natural, -2 size, +4 mage armour, +4 shield)

hp 297 (22d12+154)

Fort +20, Ref +13, Will +18

DR 15/magic; Immune acid, paralysis, sleep, SR 27

OFFENSE

Speed 60 ft., fly 200 ft. (poor), swim 60 ft

buff Speed 90 ft., fly 230 ft. (poor), swim 90 ft.

Melee bite +32 (4d6+16 plus 4d6 acid), 2 claws +31 (2d6+11), 2 wings +29 (1d8+5), tail +29 (2d6+16)

buff Melee bite +34 (4d6+16 plus 4d6 acid), 2 claws +33 (2d6+11), 2 wings +31 (1d8+5), tail +31 (2d6+16)

Charge +36 (16d6+16)-Vital Strike

Space 15 ft.; Reach 10 ft. (15 ft. with bite)

Special Attacks acid pool (50-ft. radius), acidic bite, breath weapon (100-ft. line, DC 28, 20d6 acid), corrupt water, crush

Spell-Like Abilities (CL 22nd)

At will—darkness (100-ft. radius), insect plague, plant growth

STATISTICS

Str 33, Dex 10, Con 25, Int 18, Wis 21, Cha 18(22 buff)

Base Atk +22; CMB +35; CMD 45 (49 vs. trip)

Skills Fly +17, Handle Animal +26, Intimidate +29, Knowledge (arcana) +29, Knowledge (history) +29, Knowledge (geography) +29, Perception +30, Spellcraft +29, Stealth +23, Swim +44

Languages Common, Draconic, Giant, Goblin, Orc

SQ speak with reptiles, swamp stride, water breathing