I was thinking about this a little bit, recently. The new Genestealer Cults codex ends their current timeline of activities with the Conduit touching down on Ziaphoria, instantly being enthralled by the tyrannoformed "planet" and then sending out as many missionaries as possible to spread the "Creed of Tiamat". Ziaphoria is so ferociously defended by the Tyranids that the Imperium abandons it entirely and declares the entire territory as Quarantine Extremis. Only the Deathwatch at Haltmoat, joined by the exiled Inquisitor Kryptman (who must be, like, hundreds of years old by this point) have any sense of the situation and their theories are literally as wild as the night is dark.
The only thing that is really known is that Ziaphoria has been tyrannoformed into a pulsating psychic resonator, so here are my most recent thoughts: Ziaphoria is actually the first step in a real-time galactic-gambit that will see the birth of the next evolution of the Tyranid race.
Ziaphoria is either:
A) Intended to last indefinitely. [The Psychic Telephone Theory]
-or-
B) Is going to serve a single purpose and then die. [The Cradle Of Rebirth Theory]
In both theories, Ziaphoria is, essentially, a planet-sized psychic telephone. Why? It will become the only way to communicate with every single Tyranid in the galaxy at [roughly] the same time. The Tyranids permeate every plane of the galaxy in numbers and distances far too great to communicate effectively with each other on a fleet-by-fleet basis. Using simple biological means such as following pheromones trails through olfactory bulbs or following trace amounts of infrared radiation through the vastness of the galaxy is, simply, a lost cause. Literally anything could happen that would completely disrupt that method of trying to physically communicate with each other.
It has been noted in that Tyranid hive fleets can share information between each other. The problem is the information must be shared biologically (exchanging of genetic material), which is almost the most inefficient way to communicate in the history of ever, or through the Hive Mind (exchanging of mental material). The Hive Mind connection is short-range, though. For example:
>Kronos can't talk with Jormungandr because it is distal to Kronos.
>Kronos can talk with Leviathan because Leviathan is proximal to Kronos.
>Kronos wants to talk to Jormungandr (distal to Kronos), so it first must talk to Leviathan (proximal to Kronos and Jormungandr).
>Leviathan now tells the message to Jormungandr.
>Jormungandr now shares the response with Leviathan.
>Levithan then shares the response with Kronos.
The point that I'm trying to demonstrate is that this form of communication is horribly inefficient and that the original message could easily be misinterpreted or misappropriated. It's also an extremely poor choice of communication when almost every other hostile race in the galaxy can communicate infinitely (by comparison) faster than you.
[The Psychic Telephone Theory]
Imagine, then, if you will, a planet-sized psychic switchboard that allows all of the hive fleets to make calls to each other almost instantly. Heretic psykers or ruinous daemons thwarting the advance of your hive fleet? Call Kronos. A seemingly superior Ork Waaagh! making camp on a potential prey world? Call Leviathan. The result is the same in any case: Exchanging information efficiently and quickly. Sharing information wins and loses conflicts on any scale and I doubt the universe of Warhammer 40,000 is any different. In fact, communication is so important that the Tyranids completely block out all psychic communication via the Shadow In The Warp when they are sufficiently close to a prey world. Ziaphoria will act as a psychic call relay or an Astronomicon, of sorts. In this theory, Ziaphoria is extremely important to the Tyranids and must be defended at all cost for an indefinite amount of time.
[The Cradle Of Rebirth Theory]
This theory posits that Ziaphoria is a psychic telephone, but only for a short amount of time. The idea here is that the Tyranids are too splintered to continue being an effective apex predator. If you spread yourself too thin, you lose. In this case, Ziaphoria calls the hive fleets from across the galaxy to it. The hive fleets are compelled to heed the call and pull out of every single major or minor engagement they are involved with and make haste across the galaxy to Ziaphoria.
If Ziaphoria is already too ferociously defended by one single hive fleet, imagine every single hive vessel in the entire galaxy swarming and orbiting about the territory surrounding Ziaphoria... it would be impenetrable by any mean imaginable.
Now that every vessel of every hive fleet is in close physical proximity, they share information... through both psychic and biological means, on the surface of Ziaphoria itself. Use your imagination here. It's gross. If hive fleets can "suck up" bio-mass, then they should be equally able to "spit out" bio-mass and therein lies the true gambit; the entirety of the Tyranid race will be vulnerable while Hive Fleet Tiamat defends Ziaphoria. Perhaps just one or two hive fleets will remain waiting in orbit. Perhaps the primal Hive Fleet Ouroboros? Perhaps even Hive Fleet Tiamat, itself?
The whole planet will become a writhing, massive ocean of the organic, super-rich bio-mass hailing from every hive fleet regurgitated into one location; the entire genetic legacy of the Tyranid race, every original Tyranid strategy, every original Tyranid evolution, everything at all that is, was or will be a Tyranid is here in this genetic soup. Even the hive ships themselves submerge themselves into the nutrient-rich slurry and dissolve away to become part of it.
And then the last hive fleet remaining, maybe Ouroboros, begin to drink it up like ambrosia from the gods. The hive ships swell to unprecedented sizes and then begin to rapidly gestate. The Tyranids are still vulnerable on all fronts and Tiamat must hold the lines, so-to-speak, at all costs; to lose now would mean the extinction of the Tyranid race. The gestating vessels, so empowered with with the super-rich bio-mass, begin to calve and it is in this very moment that the doom of all things is born: A new Tyranid hive ship imbued with the perfect understanding, immutable hatred and unsatiated hunger for all that the Tyranid race has ever known. The new Tyranids are, literally, perfect killing machines and within a short amount of time, the new Tyranid hive fleet bears countless new vessels and, ultimately, consumes every single part of Ziaphoria. This new hive fleet then devours the hive fleet that bore it, and even Hive Fleet Tiamat surrenders to the new hive fleet and are themselves consumed. The new hive fleet effortlessly devours everything surrounding the Ziaphoria system.
Then, they begin to seek out the remaining extant races for consumption...
