Eridonian Authorities had approximately 3 local system hours (ES) notice concerning the catastrophic wave of devastation brought upon Aquarius by the Aftershock; as it decimated the outer worlds, making its way to the Core Systems and the very heart of the Old Federation.
The initial response was to initiate a planet-wide Federal Emergency, activating the Federal Emergency Services - and Eridonian Planetary Guard military units - to restore order as civil unrest broke out, as news of the Fall of the outer worlds swept through Eridonian media platforms. The Eridonian Federal Government, the seat of power of the entire Federation, instructed local planetary governments among the myriad districts of the Eridonian City Tiers to enact an Evacuation Protocol created during the middle of the 4th millenium as a contigency plan should the capital world ever be attacked by weapons of mass destruction during the Phoenix Unrest.
The protocol required a mandatory evacuation of all citizens to subterranean Shelters specifically designed and hardened to resit atmospheric-damaging weapons, or major stellar events. However, despite the relatively cordial response of the Planetary government; unrest quickly spiralled out of control and masses of civilians scrambled to get off-world, after news of planet-wide destruction came in from the first of the inner systems.
Major city-wide riots broke out, requiring direct intervention from the Eridonian Planetary Guard to assist the Federal Police in attempting to maintain order. Within an hour, the space-ports across the Upper Tier Districts were over-crowded with terrified civilians seeking a vessel to take them off world. Their efforts were thwarted by a planet-wide blockade of atmospheric entries and exists, enacted by the Federal Navy operating over Eridonia, on instruction of the Planetary Government.
The EPG and Federal Police resorted to live-fire lethal response over the next two hours as chaos ensued and the Federal Emergency order broke down into anarchy. EPG armoured response units were deployed in the streets and Federal Police ARV units carried thousands of heavily armed Armed Response officers into the fray, in a desperate attempt to quell the riots and get people to the apparent safety of the Tier 0 Shelters.
During the chaos, there were reports of unidentified attacks taking place in the lower city tiers, focused around Tier 0 and Tier 1; at the very bottom of the stacked-city complex, several kilometres from daylight. The first reports indicated some Federal Police units had sustained heavy casualties from assailants described by response officers as 'aliens'. The reports soon became credible and the Eridonian Planetary Guard re-focused its efforts on securing the lower tiers after thousands of civilians were reported as missing from the evacuation attempt.
Shortly before the Aftershock hit Eridonia, the riots were almost under control. The Federal Police and EPG had blockaded essentially all major Tier Elevators to the upper districts and prevented rioters from moving between Tiers, effectively cutting off their attempts to organise. However, the reports 'unknown assailants' in the lower tiers eventually became so severe that entire Planetary Guard units were reported missing, including armoured vehicles.
It became clear to the Planetary Government and Military Command that Eridonia was under a hostile invasion, but before any major organised response could be committed; the Aftershock ripped through Eridonia's atmosphere, sending lethal Polarytic Radiation radiating across the upper tiers. The initial blast wave disabled electronic systems and shut-off the Magneto Tectonic Stabilisers between the vast City Tier pillars; almost immediately causing city-wide tremors that brought down many free-standing buildings on the upper tiers.
Communication was broken almost immediately, and the entire city went into an electrical blackout as Federal Navy warships operating over the city in support of the Police and Planetary Guard came plummeting to the surface as their gravimetric stabilisers shut off. Many of the Districts of Tier 4 were completely destroyed by falling warships, mostly of the Farlease-class, a two-hundred-thousand ton cruiser warship operated by the Federal Navy as a patrol vessel, including in-atmosphere. The impacts caused more tremors which damaged vital infrastructure on the lower tiers that were somewhat shielded from the harshest Polarytic Radiation exposure.
Eridonia fell within a few hours, without environmental regulation and tectonic stabilisation, the relatively protected lower tiers became graveyards for millions of civilians, suffocating on toxic fumes from the detonations above or failing infrastructural systems below.
The Aftershock hit so fast that there was no official record of the 'alien' attacks in the Eridonia Archives. Such data was only discovered several hundred years later by ATIS operators combing the ruins for relics as part of the thousand-cycle clean-up operation of the Grave World of Eridonia.