Help with CR for my encounter

[Rycke]

I'm planning an encounter for a group of 4 lvl 6 players and wanted to get an idea of whether it's too easy or too tough and what CR to give it.

The encounter starts with a giant raven that is the equivalent of a juvenile black dragon. On the first round of the encounter it breaths a stream of smaller ravens (instead of acid) at the group for 8d6 damage as the dragon breath weapon. This leaves behind a carrionstorm swarm with 12 hp that acts as normal (auto 1d6 damage when occupying the same square as a character). At this point, the raven becomes as a young black dragon. Until 1d4 rounds later, it attacks as a young black dragon and the swarm is also active. Then, it breathes again for 6d6 damage, leaving another swarm and becomes as a very young black dragon. It continues to spew swarms, becoming one age increment smaller, until it is all gone. 1d4 rounds later, it will coalesce again into the juvenile black dragon sized giant raven. The only way to kill the creature is to kill the swarms left behind. Each swarm killed before it coalesces again reduces the age increment by one when it re-coalesces.

Hopefully my explanation makes sense. Thanks for your consideration.

[Malachandra]

Wow, that's an awesome idea. Seems like it will be a tough (APL+1) encounter until the second swarm hit, then it will be progressively easier. The swarms won't be much of a threat to a lvl 6 party except for their auto-damage. After the second swarm, I'd expect the party to kill the dragon/remaining swarms pretty quickly. What happens when they kill the dragon? Does it die for good, or dis-corporate/re-corporate a few rounds later?

Either way, sounds like it'll be on the easier side. If the party can last the first 1d4 rounds, they should have no problem after that. It'll be very memorable though. I'm curious as to how it goes.

[Dasrak]

One thing I will mention is that it's very unlikely this cycle will come full circle, requiring 5d4 rounds (average 12.5) before it hits its reform step. Carrionstorms have only 11 hit points and 12 AC; the party would have to be exceptionally weak for such a frail foe to survive even a single round. Meanwhile the giant raven is just becoming weaker every time it creates one of these weak swarms. And that's presuming the main raven even survives long enough to get off all its swarms; as it loses power it loses hit points and becomes easier to hit.

You might want to consider choosing a more powerful type of swarm, and also having the giant raven completely dissipate earlier rather than assuming extremely weak forms that can be eliminated easily.

[Rycke]

Thanks for the advice.

Malachandra, if they kill the dragon, the remaining swarms will coalesce back to whatever size they can obtain.

Dasrak, I think I'll make it a bit tougher. I'll keep it as Juvenile after the first blast, make the swarms 2d6 damage with 16 hp and 14 AC, and the smallest it will get is very young size. I think that should make it sufficiently challenging.

[Dave Justus]

So any damage done to the 'main' creature is totally ignored? This makes it quite a bit tougher of an encounter, especially if there isn't an easy way to realize/discover this.

That also means that the longer it takes between the initial breaths the more dangerous it is. If it has 4 rounds to attack as a young black dragon while invulnerable it will be a lot more dangerous than if it only gets one, something that isn't true with a real dragon as usually it is somewhat of a toss-up whether attacking or breathing is more dangerous.

Beyond any physical attacks, it is impossible to avoid at least one breath at each category (so 8d6, 6d6, 4d6 and 2d6) that is going to be about 100 points of damage, obviously enough to kill just about any 6th level PC absent healing, not counting any other damage. Of course one PC might not get hit with all 4 breath attacks, but I would expect that to happen (and a couple to get hit multiple times.) Depending on how the die rolls out, I would expect healing resources to be pretty well exhausted.

I also strongly doubt that it will ever 'reform' after the last breath. Even if the party had totally ignored the swarms up to this point, I would expect a single round of actions to be able to destroy 4 carrionstorms.