The Last Day was a short period of armed conflict initiated by the unnamed zebra empire against the pony nation of Equestria, during which both nations released their highly-destructive megaspell payloads against each other.
The megaspell onslaught lasted mere hours. The first city to fall was Cloudsdayle, which immediately resulted in the pegasi's abandonment of the war effort and their subsequent secession from Equestria. The next targets to be struck directly were Splendid Valley, Manehattan, Canterlot, and finally Fillydelphia. After this point, the pegasi raised a near-permanent cloud curtain over all of Equestria, blinding the zebras for the remainder of the megaspell assault.
The conflict marked the conclusion of the Great War and resulted in the destruction of much of the known world and the dissolution of Equestria, its territories reduced to what are now known as the Equestrian Wasteland.
The attack, according to canon, was a complete surprise for both the Equestrian military command and civilian government. No advance warnings of any sort were provided; the early warning systems either didn't exist at all, or for some reason were completely unable to at least convey an "air alert" to the population.
Destruction of Cloudsdale
The city of Cloudsdale was obliterated instantly by the destructive power of the megaspell. Ponies outside of the city's boundary were also killed by its radiation and energy. Some, such as Ditzy Doo, were (un)lucky enough to be turned into ghouls by the radiation.
As a result of Cloudsdale's destruction, the surviving pegasi closed up the clouds and retreated into their remaining cities, abandoning Equestria to its fate.
The destruction of Cloudsdale was the only confirmed air blast in canon. The equivalent of weapon could not be determined because of insufficient data, but was big enough to produce significant devastation on the surface below. The long-therm radiation contamination probably wasn't significant, because of abscence of direct contact between the megaspell and surface.
Destruction of Maripony and Splendid Valley
Although it is widely believed that Manehattan was the second target to be hit by a megaspell, Splendid Valley was hit moments earlier. We know this because Pinkie Pie tried to send a message to Twilight Sparkle but was unable to because the Maripony site had been destroyed.
The balefire bomb was planted somewhere within the subterranean tunnels of the Maripony facility. During the destruction of Maripony, Trixie fell into a vat of taint. She became the goddess and absorbed Twilight Sparkle, among other ponies.
The Splendid Valley was the only confirmed attack against hardened underground target in canon. The blast was either surface or underground. The complex wasn't completely destroyed; that means that either the weapon equivalen was unsufficient or the ground zero was placed incorrectly. No additional attacks were confirmed.
Destruction of Manehattan
Manehattan was the third target to be destroyed by the Zebra forces. Before midday, a team of zebra infiltrators and members of the Four Star rail company armed a Megaspell device within the city. Attempts by the Ministry of Morale were made to stop the infiltrator team but they were unsuccessful in their mission. The megaspell was detonated killing nearly all life within Manehattan, including Pinkie Pie.
The destruction of Manehattan is chronologically the last recorded megaspell detonation. The timing of further megaspell detonations has no significant bearing on events within the story.
The destruction of Manehatten was caused by single surface blast of megaton scale. The one detonation was sufficient enough to destroy nearly all population and make a significant material damage; the weapon equivalent could be assumed as megaton scale. Some hardened structures and shelters were able to whistand the initial attack. No additional attacks were confirmed.
Destruction of Canterlot
In the hours (and possibly days) leading up to the other megaspell events, Canterlot had been under constant barage from long range Zebra missiles. In order to keep the city from being destroyed, Princess Celestia and Princess Luna surrounded the entire city with a protective shield that was impenetrable to anything including air. Nothing could pass through the shield in either direction, and the city was eventually annihilated by the necromantic Pink Cloud that was released from within. Before the shield collapsed, signalling the death of the last Goddess Sister that was straining to maintain it, it was said that the city looked like a solid pink bubble. The vast majority of Canterlot ponies died during the attack, including Rarity. The few who survived however, were turned into Canterlot ghouls.
While the original Pink Cloud was generated in a similar way to how a Water Talisman or Hydrogen Talisman (for Pinkie Pie Balloons) works, the dragon that guarded the royal treasury ate the gemstones in the device and began snoring out Pink Cloud instead of smoke. This would make Canterlot a permanently environmentally hostile area despite decades of diffusion and rain.
Though the city of Canterlot was left largely intact, the Enclave would destroy it some 200 years later in the months following the opening of Stable 2.
Canterlot was put under a prolonged barrage of megaspell-armed missiles, in an attempt to either destroy or isolate the capital from the rest of the country. The attack, due to the lack of damage to surrounding territory, was probably on the kiloton-scale. This is the only canon-confirmed case of a prolonged attack and subsequent strikes.
Destruction of Fillydelphia
Fillydelphia was struck by a megaspell-armed missile during the general bombardment, as opposed to the previous sites, which were destroyed in preemptive strikes. The detonation created a huge, radioactive crater in the city center. Apple Bloom passed away sometime during the attack.
The Fillydelphia attack was from a surface or penetrated blast from a megaton-range weapon. The detonation caused extensive damage and created a large, highly radioactive crater. The long-term balefire contamination was probably significant due to the fallout of radioactive materials from the blast.
Destruction of Whitetail woods
A large number of megaspell-armed missiles were destroyed over Whitetail woods. The winds carried the radiation and poison of those weapons across all of Whitetail Wood, covering Equestria’s once beautiful forest and poisoning the reservoir all the way to the edge of Ponyville, causing even the ground in Sweet Apple Acres to be sick and poisoned.
The blasts over the Whitetail woods was probably the small-scale surface detonation of relatively low equivalent. There is no indication of widespread destruction of vegetation or large cratering. The long-term radiation contamination was very significant.
At least some Equestrian Megaspell arsenal was used in retaliation strikes against zebra's. The only known use is the counterattack provided by "Celestia Prime" long-range solar-powered beam weaponry, that was used directly from the Tenpony Tower by the Ministry of Arcane Science civilian personnel. According to the canon information, the weapon were used against pre-determined locations, that it personnel assumed as the zebra missile bases. After Pegasi closed off the skies 30 minutes later, the solar-powered weapon became useless.
The attacks of the "Celestia Prime" were the only canonically established retaliation strikes by Equestria. Their effect and targets remain undetermined by canon.
The megaspell exchange was, basically, the last significant military action of the war. The civilian and military infrastructure of Equestria (and, presumably, the Zebra's Empire) was almost completely destroyed during the missile attacks, and casualties counted in tens of millions, far beyond the nation-scale survival treshold. The decapitation of both civilian government and military command, degradation of communication&control network and collapse of local authority made any large-scale organised relief actions impossible, which - combined with radioactive contamination of Equestria territory, food and water sources (and, also, climate changes, induced by the Pegasai cloud screen) led to the extinction of more than 90% of total population.
It's unclear if any kind of "broken-backed" warfare occured after the megaspell exchange.
It must be noted, hovewer, that large part of casualties was indirectly caused by inactivity of the Pegasai Enclave, which basically separated itself out of land-based Equestria on early stage of megaspell exchange. After the megaspell exchange, the Pegasai still mantained effective organization, have their communnication and military capabilities largerly intact, and were capable of providing at least basic relief works on surface. Instead the Enclave govenrment adopted the policy of almost total isolation from the surface, denying the land-based survivors of any help and mantaining the cloud screen over Equestria. The resulting "nuclear twilight" effect almost completely ruined the remaining ecosystem, made agriculture on the surface ineffective, and hampered any civilization-rebuilding attempt that may be conducted on surface.
- ↑ The reason was not explained in canon; either Zebra's missiles have some sort of optical terminal guidance (and pegasai hope to reduce the accuracy of missile strikes), or the main reason was to deprive zebra's of post-attack confirmations, and force them to spend additional missiles on the already-hit targets.