Spoiler
Show
A major disagreement within the Culture begins as additional losses of citizens and even another Ship results in a more serious attitude towards Chaos. Eventually, the more hawkish faction wins out and imposes a form of internal control (in the name of maintaining technological advantage).
Any and all actions leading to the ending of Chaos as a threat are pre-emptively approved. This includes xenocide, although to be used in extremity.
Year 1
A number of Necron Tomb Worlds are dismantled and technology confiscated. The Eldar begin to break away as the farseers forsee hostile relations with the newly reformed Culture.
Contact is dropped with the Tau. Tyranid exterminations are halted. Instead of informing the IoM of Chaos cells, the cells are just summarily destroyed instead.
Experimentation with warp technology begins in earnest, using disposable planets in uninhabited systems.
Year 2
Tau warp-dimness is reverse-engineered and a slightly more effective version is put in place for all Culture citizens. The requisite genengineering technology is gifted to Mars, Inquisitorial factions and anyone in the IoM who is interested.
A major Ork Waagh begins to build against some IoM planets, drawing in a huge number of Orks. This Waagh is influenced, although not directly led, by the Culture who provide them with limited hyperspace drives (and principles of manufacture) that they are able to divert to more juicy IoM targets.
When the now-unstoppable Waagh reaches epic proportions, a huge amount of IoM planets in Segmentum Obscuris have been laid to waste, the Waagh is directed into the Eye. It easily overruns Cadia and the Orks enter the Eye to do battle with Da Spikies.
Near the end of the year, the tyranid hivemind connections have been reverse engineered and majority Culture organics become hivemind nodes, directly connected with each other, as the new fad (encouraged and abetted by the Minds) takes hold. GSVs cast a Shadow similar to that of a small splinter fleet.
Year 3
Orkified CREWs and plasma weapons allow the Orks to gain a foothold in the Warp, successive battles are drawing more and more orks into the Eye, the war machine fed from the overrun IoM planets that the Culture cultivate into Ork growing farms.
Necron atomic scale technology is perfected. It affords new interesting weapons (eg. a subatomic scale wire that causes fission in heavy metals it passes through; as well as far-gamma ray CREWs with a power density roughly two orders of magnitude higher) and a massive scale reduction for Culture electronic equipment (now called femto-electronics), allowing previously palm-sized drones to now fit into something the size of a knife missile.
Additionally, production of monolith fields are now possible although adverse reaction with the hivemind network discourages its production.
Year 4
IoM response is too few and too late. Exterminatus is carried out on a few Ork infested worlds but the Culture wipes out the fleet and restores the planet by terraforming it.
Chaos pushes back a little against the endless Ork flood into the warp but likewise, it is also too few and too late. Femto-scale weapons in the hands of the orks begin to leak over into Chaos but the now massive Ork Waagh field can produce femto-scale technology without the requisite base which Chaos does not have. Armour plates with the density of a neutron star are used as common armour on Ork tanks and Orkish bones, enhancement surgery being routine in this Waagh, can support an entire mountain without bending. The tanks can withstand pressures at the core of a gas giant and are immune to non-femtoscale weapons.
The Culture develop a better CAM warhead, instead of merely high-density antimatter, these new warheads are anti-neutronium mixed with normal neutronium, kept separate by ultra-strong forcefields. A warhead the size of a person weighs as much as a small moon and has the explosive power of a small sun going nova.
Warp engineering takes off as the Culture manage to build their first warp device, after reverse engineering an Eldar artifact captured last year. Eldar relations are sacrificed in the name of "necessary measures". A Dark Eldar controlled webway gate is dismantled.
Year 5
The webway is reverse engineered using the new warp engineering technology. Basic reality alteration and limited psyker-like powers can be engineered in new citizens, although this is treated with great caution and not applied.
2 months into Year 5, a major breakthrough is made on the warp engineering front and a warp-based computer becomes possible. Intelligence engineering is being translated to the new understanding of the warp.
6 months into year 5, the first stable warp zone with physics favourable for computation is permanently set up near the Ork controlled areas in the Eye. The modification on a hybrid technology of gellar fields and necron pylons not only maintains warp connection but actually conducts reality alteration to increase the speed of light limit and accelerate time; additionally, hyperspace still exists inside this zone.
7 months, a warp-hyperspace hybrid Mind is constructed, with processing power nearly 6 orders of magnitude higher than the best normal Minds can achieve.
8 months, future sight is independently discovered and the Ork offensive suddenly gains traction. The Chaos gods begin to act offensively for the first time.
8 months 15 days, all Culture ships have been retrofitted with stabilized-warp technology, effectively, Culture ships now operate more efficiently IN the warp and have autonomous warp-mines that ambush the daemons attempting to attack them. These mines were reverse engineered from daemons captured using reality bubble traps, in some ways, they resemble the daemons themselves.
9 months, all Culture Minds are warp-hyperspace hybrids. Further understanding of reality alteration results in an effector-like array that projects bubbles of a desired reality. The Culture ships take over the Ork offensive against Daemons, the orks themselves are reality altered into containing a Tyranid hivemind and are connected into the Culture hivemind grid. Effectively, they are assimilated into the Culture as citizens and are personality-modified under the new reality bubble towards Culture standard.
9 months 10 days, a number of the most powerful Culture Minds, in the face of direct attacks from the Chaos gods, begin to enact a modified Ascension process utilizing the Warp engineering technology. They use the hivemind-waagh hybrid to do so and the resultant psychic amplification instantly destroys all psychically sensitive Eldar (which is almost all of them) as well as every single psyker of every other race. Only the Necrons are unaffected.
The gestalt consciousness of the hivemind has reality alteration capabilities rivalling that of the Chaos gods, of whom, Nurgle has been systematically depowered by intrusive Culture campaigns amongst the IoM and Slaanesh is on borrowed time due to the sudden dearth of Eldar. Only Khorne comes close, but not close enough.
---> Scenario A: The Ascension gestalt consciousness wins easily
After the defeat of the Chaos Gods, the gestalt absorbs the aspects it wants as well as those of the Eldar gods. Unlike the old Chaos, the new "Chaos God" retains the ability to act in reality to a large extent and a self-replicating swarm of probes is seeded across the entire galaxy.
The Necrons, sensing the plan, resist. Futilely. Shortly afterwards, the entire galaxy is placed under a reality-alteration bubble. The remaining Necron strongholds are attacked, and when C'Tan shards are used, nova-bombed.
---> Scenario B: The Ascended consciousness wins after a long brutal campaign
The initial battle goes poorly and the new consciousness is forced to take extreme measures to win. Firstly, all direct conflict is conducted in contrary fashion to the methods that power Khorne; all conflict between the IoM and current xenos are abrogated by reality bubbles.
Secondly, all Culture organics are uploaded into inorganics, the loosed soulpower prevented from powering the Chaos Gods by a massive "Wall" across the warp. This is still insufficient power and the soul formation process is hastily re-engineered in a time bubble before being mass-implemented.
The sudden spike in gestalt psychic power afforded by the continuous soul drain, now re-termed as a warp-gathering mechanism shatters what remains of the galaxy and every single warp-sensitive sentient, including even the Tau, are killed from the backlash. The corresponding loosed soul power increases the turmoil in the warp to such an extent that the Eye expands to engulf the entire galaxy.
The new War in Heaven rages for some time, but eventually the Chaos gods are defeated by extensive and chronic application of creation/upload cycles. The uploaded population of the galaxy now exceeds the number of atomic particles in the known universe, most of whom existed for only days by Warp-standard-time before the war was over.
---------------------
Epilogue (from both endings)
A cyberspace-level 7 historian examines records of the Second Great War and notes parallels between the Culture's final actions and those of the original Chaos. Indeed, depending on the definition, the current "Culture" behaves much like the prior Chaos did in the essence of the term.
True, the New Chaos is not divided and the gestalt feeds off raw soulpower instead of aspects, and the New Chaos is not structured off hostile power relationships. Culturally, New Chaos is nothing like the old.
But the actions taken to win that conflict could be said to have defeated the point, countless numbers of sentients were sacrificed and an impossible-to-estimate amount of potential culture was lost. The only thing that remains is the Culture.
The Culture survived... only by becoming its enemy.