General Horner
I must apologize for the brevity of this note, but we have limited time to compose our messages and limited space to carry them. Accordingly, I'll try to keep this brief, and hope it can provide some useful intelligence on the nature of the Zerg. The Protoss have a serious advantage on us-they've been able to tell us a lot, especially the 'dark templar' Zeratul. The Zerg are also a psionic species and the Protoss have been eavesdropping on their communications to a limited extent. More importantly, Zeratul and his attendants have killed several members of a key subspecies and been able to gain access to their thoughts and memories.
The Zerg are a hive mind, as we guessed, but they are far more than just that. The Overmind, the creature we helped kill on Aiur, was apparently the pinnacle and supreme authority of the Zerg minds, singular and directing the entire swarm. It was, however, apparently killed for good, with no chance of it coming back. However, it was not the only mind within the Zerg. Next down the ladder are the Cerberates, creatures that like the Overmind, are almost entirely creatures of thought and psionic power, being physically incarnated as giant pale worms. I've enclosed a few pictures, apologies for the poor quality. The Dark Templar can kill them with some kind of 'void energy' but I'll be damned if I have any understanding of what that is. Without that, they seem to reincarnate quickly, growing a new body in just hours or days. Still, there is some good here-they were apparently dependent on the Overmind to reincarnate, and without that, maybe they can't come back. Even if they can come back, killing them seems to have a negative effect on their hives for a few hours until they get over the trauma of dying. However, the next steps down the chain of command-the Overlords, Queens, and to some extent Defilers, seem to maintain control of limited zones of the Swarm as well, serving as field-officers of a sort. From what we've seen on Aiur, however, if their Cerberate is killed, these are themselves traumatized and revert to a certain level of instinct. While the lower reaches of the swarm might be capable of more thought, they aren't used to or designed to exercise it, so a Brood goes feral without direction from above.
Of the named Broods, we can confirm that three have been decapitated-Garm, Baelrog, and one we have dubbed Grendel. The Zerg themselves apparently wiped out Garm, but Baelrog and Grendel are still active and still very dangerous, abliet lacking strategic direction. Tiamat was present on Aiur, but it's Cerberate is apparently still on Char while the Overmind was in direct control of the brood here. But there's so many Zerg still on Auir that even killing the Overmind hasn't changed the basic calculus and Tiamat still has Daggoth. That's the name of the Cerberate in control apparently. They apparently have names...and personalities, strange as that is to think. The Overmind apparently sounded grandiose and formal. Its death throes didn't feel very grandiose, but I'm no telepath.
Kerrigan seems to sit somewhere to the side of all this, we're not quite sure what's going on there. We thought she was the Overmind's champion, her conversion the ace in the hole the Zerg needed to create some kind of super-Zerg that could fight the Protoss one on one. But she seems to have never been on Aiur, so who knows what the story is there. She might not have direct command of a Brood, but I think it's safe to guess it's just a matter of time if she decides she wants one. Kerrigan's got a leg up on everything else inside the Zerg hive-mind at the moment, and that's being used to thinking for herself. If she can't turn that into forcing her will onto Zerg, and leveraging that, then I'll be surprised. While I cannot give you any more actionable information, she might move more early than would be wise-she's very confident in her own strength now, perhaps overconfident.
I close now, wishing you find this information useful. May it save lives or give you some clarity in a troubled universe.
Your obedient servant.
Captain M. H.