I'm just a bit vexed because I'd been working on stat blocks for volcanic dragons for...personal reasons, lets say. They briefly mentioned these in Monster Manual, or was it Draconomicon? Anyway, once I heard about these "catastrophe" dragons I started kicking some mechanics around because I wanted to have a solid foundation setup for when I would need them at the epic tier of my campaign. That being said I'm not really upset because, well, they did the work for me and probably a deal better that I did. Of course, I could also pitch them my ideas since it is a playtest.
Volcanic dragons work on removing fire resistance/applying fire vulnerability. Their aura and basic attacks all work on eliminating this, obviously because they can setup a devastating burst attack. You see, while their aura starts out pretty tame, they can expand it twice and then unleash a very wide area burst attack that deals ongoing fire damage, but thankfully reverts the aura back to normal. However, there is no limit to this so they can built it up again and again. The only silver lining is that its a standard action to expand it.
Now, they dont have a breath weapon. Their expanding-exploding aura fills in that niche. However, they can also create lava vents as a minor action. They can only do this once per round, and it recharges on a 6, so...yay? They dont have a duration, so if the DM rolls well you can count on a good chunk of the battlefield becoming hazardous terrain.
And thats just the little guys. Older categories dont really change much. The ongoing damage/vulnerability scales up for the elder volcanic dragon, but otherwise nothing new gets thrown at you. Until you hit the ancient category, at which point it can also create fissures of lava (kind of like a wall of lava) and their aura over doubles in size.
I look forward to playtesting this tomorrow. They are only elites, so I can add in some fire-themed allies into the mix and see how well that messes with things. They are really good at negating fire resistance and dealing ongoing damage at the same time, so I'm concerned that it might be very overwhelming when paired up with other monsters that have fire attacks. But, thats what a playtest is for!