Fall of Insomnia

The Fall of Lucis is the first major event in Final Fantasy XV. The battle occurs after a fake peace treaty drafted by the Niflheim Empire in order to access the Lucis Crystal as part of its bid for world domination.

Prelude
A prior attack had occurred on Founder's Day of M.E. 734 by the Empire via the newly-released Adagium, rechristened Niflheim chancellor Ardyn Izunia, as well as Imperial researcher Verstael Besithia, with Izunia being dispatched to disable the Wall amplifiers to ensure the attack. Although the wall weakened, it wasn't sufficient enough to breach Lucis' defenses, and Izunia also went against orders in order to force King Regis Lucis Caelum to summon the spirit of Ardyn's brother, the founder king Somnus Lucis Caelum, to settle things and disappeared due to divine intervention. Because of this setback, Niflheim was forced to adopt another strategy.

Fall
That opportunity eventually presented itself around May 16, M.E. 756, when Niflheim requested that it and Lucis sign a peace treaty to halt hostilities, with Izunia and Emperor Aldercapt personally overseeing the treaty signing. In reality, the treaty request was a feint in order to have Niflheim's Magitek infantry infiltrate the kingdom and destroy the crystal powering the wall using information provided by Lucian rebels. One of these rebels, General Glauca, was in actuality Titus Drautos, the commander of Lucis' royal guard, the Kingsglave. To make matters worse, several of the Kingsglaive ended up defecting to Niflheim. Niflheim also attempted to abduct Lunafreya Nox Fleuret, the Oracle of Altissia, although Nyx saved her both from Niflheim and his turncoat comrades. Regis suspected the treaty was a trap, but ultimately allowed for it to happen, both because the crystal demanded for one more life to be forfeited, and also because he was unable to maintain the wall for much longer. Luche Lazarus ultimately ended up killed from being deemed unworthy by the Ring of Lucis after Lunafreya tricked him into wearing it.

King Regis attempted to fight off Glauca, although he was ultimately killed in the attempt, although not before allowing Nyx and Lunafreya to escape. Nyx ultimately is forced to use the Ring of Lucis to stop Glauca as well as erect the Old Wall to repel the daemon assault by the Empire. Lunafreya was evacuated from the city before then.

Aftermath
Aldercapt and Izunia left the city with the crystal in tow, having succeeded in their overall goal. The attack was unexpected enough that it made headlines in various newspapers the next day. The radio news broadcasting this also initially reported that, aside from King Regis, the crown prince Noctis Lucis Caelum and Lunafreya had also been killed in the fighting.

In actuality, King Regis had deliberately sent his son Noctis to be wed to Lunafreya at her hometown of Altissia to get the latter to safety from the attack, suspecting that the latter was the prophesized True King to bring the world to light, and thus wasn't even in the Crown City at the time of the attack.

After the attack, a checkpoint was established within Insomnia by the Empire. Noctis, upon learning the news from his caretaker Ignis Scientia while being forced to stay at Galdin Quay due to the Empire imposing a blockade at Altissia, went back alongside Prompto Argentum and Gladiolus Amicitia to investigate, wiping out several Imperial infantry when bypassing the roadblock.

Cor Leonis alongside Monica Elshett and Dustin Ackers helped evacuate several of the Lucians from Insomnia during the attack, including Iris Amicitia, and are among the few survivors. Cor contacts Noctis shortly after the attack and confirms Regis' death, and eventually reveals that Regis had Noctis leave ahead of time before having Noctis adopt his role as king.

Lunafreya learns from her handmaid of Nyx's passing, and is supplied his hairpin.

It is later revealed that Izunia had his own motives for conducting the attack, as, being the demonized brother of the Founder King, it served his agenda of ending the Lucis Caelum line as his revenge.