After the evolution master's rebellion and establishment of diplomatic relations with the Daelaam and the Terran Dominion, there was a need for zerg that would be the Overqueen's heralds on the border worlds and especially the DMZs. Realizing that the Broodmothers are critically important for the Swarm and the Queens are not intelligent enough, the Overqueen Zagara decided to create a new strain.
Because of Abathur's betrayal, she did all the genetic work by herself. Fortunately, she didn't have to invent anything radically new because all the genetic stamps already were in the hivemind. By using the Broodmother's genetic sequences (most likely her own) she changed it according to her desired image of the zerg that would be fitting for such a specific task.
A new strain have high intelligence and a flexible mind, a calm nature, and developed psi-speech centers. However, they have reduced ability to control the minions (because the Broodmothers could see them as a threat to themselves) so their own broods are pretty small numbers (in zerg measures, ha-ha).
Having a height of about 3 meters, they're a bit smaller than even the Queens and much smaller than the Broodmothers.
Being the Overqueen's personal heralds, they serve the local Broodmothers and authorized to negotiate with the Daelaam's and the Dominion's colonial representatives.
Also, I'd like to think that they use jorium crystals to enhance their psionic link with the Overqueen to prevent the loss of communication (how it happened to Niadra).
:)












