I ran the Dragon today as is with a change from Black Dragon to Red Dragon and some fluff changes. I increased the AC to match a solider and switched fort and reflex as he would be strength/con primary. Someone noted this earlier, but it's not because wizards made his reflex higher by some decision to reduce the fort, the black dragon has dex as his primary attribute: hence the reflex.
Otherwise it was as is, just replace "fire" when you see acid. Now this is not a test. It's a real game, with real PCs and they have never seen this kind of dragon ever before. Overall, it was an amazing fight and they said it was one of the best solos I've ever run (they've been playing with me 2 years if you're wondering).
It ended with three PCs almost dying, one on 9 HP and one on 10 HP. They killed the "Red Dragon" on the very last real "last-ditch" attempt to do so. Any further and it was a TPK, though they set the last attack up well so that it was very unlikely to have any chance of missing! An incredibly intense and exciting battle was had.
Firstly, the damage effect when bloodied combined with the dragons impressive attacks is just plain brutal. The auto damage to all melee characters adjacent is really ferocious and with the vulnerability shroud it can be very difficult to deal with. Ultimately though what really made a huge impact was my amazing ability to roll critical hits: 6 in total. With vulnerability it was 25 total damage and that's a hefty amount of HP, even for the parties Warden! I downed him without almost any problem, but due to using a potion of fire resistance it reduced the damage enough so that he didn't die.
The party was (all level 3):
Goliath Wizard
Half-Orc Warden
Revenant (Gnoll) Rogue|Assassin.
Shardmind Monk
Wilden Shaman
The hybrid was the replacement for the Runepriest who met his shadowy end last session. The most interesting part of the combat was the party was so mauled, they were faced with the antagonist (a succubus who had sent the devils and the hellfire infused dragon at the PCs). She essentially got them to give her important information as to the location of a monastery in the elemental chaos. Most importantly, the two PCs who were standing at the end (the Revenant and the Shardmind) are basically on an agreement with the devil to backstab their other ally (the Efreet Quassemi). The rest of the party though, who have agreed to aid the rather dubious efreet have no idea about this particular state of affairs (as they were having a dirt nap at the end of the encounter).
So we have a potential double cross situation and it's going to be rather dramatic! I am loving how the game is going thus far, even if it seems to be rather lethal!
Returning to the Dragon though, I think I would change the auto-damage to 2 fire damage and scale it appropriately. 5 fire damage is just a bit much. One thing I think is interesting about this dragon is the "stun" replacement effect is much better than a stun. The additional fire damage is devastating, but PCs won't remove it because it uses a standard action. Effectively, they don't sacrifice attacking for removing the incredibly debilitating effect - even though this makes sense. In reality, it's the hellfire blood/acidic blood ability that made the biggest difference. When bloodied, 5 autodamage combined with 5 vulnerability becomes a huge deal rapidly. PCs won't remove the power before the creature is bloodied, so will basically get "ambushed" quite literally by it and then chaos reigns.
So I think making it 2 damage would be much better. I would also reduce the vulnerability to 3/5/10 (that would be by tier). So at most it's getting a +3 bonus to damage and the acidic blood is doing the 'intended' 5 damage. Overall though despite being an incredibly difficult fight, they pulled through and it was tense, exciting and dramatic the whole way through. This is exactly what you want out of a solo monster. The severe beating it dealt to the party even led to some interesting roleplaying and allowed me to open another entire path to the storyline up as well. So overall I was a very pleased DM.
Final HP tallies (for the curious)
Goliath Wizard (-12, Dying, 1 failed death save)
Half-Orc Warden (-18 Dying, 2 failed death saves)
Revenant (Gnoll) Rogue|Assassin. (10 HP)
Shardmind Monk (9 HP)
Wilden Shaman (-11 HP Dying, 0 failed death saves)
As you can tell, they were in no shape to fight the succubus who turned up afterwards on full HP and flying out of range of either of the current characters still standing. But why kill individuals who show such determination when you can potentially use them to your own ends? It was quite a tense conversation, but with a few agreements with the odd backstab it soon turned to the parties favor. Now I just have to think how to utilize this delicious web of betrayal.