The attack during Founder's Day is a major event in Final Fantasy XV: Episode Ardyn that happens prior to the events of Final Fantasy XV. The battle occurs after Ardyn Izunia infiltrates Insomnia during the Founder's Day festival on M.E. 734 to burn it to the ground as revenge for the treachery of its founder, his brother Somnus Lucis Caelum.
Ardyn was driven mad from reliving his betrayal at the hands of his brother millennia prior upon daemonifying the Astral Ifrit. Ardyn was made the chancellor of Niflheim for revolutionizing the empire's military, and proceeded to spread the Starscourge while infiltrating Lucian bases and daemonifying people to learn more about the kingdom. Doing this twisted him into a wicked echo of his former self. Ardyn conspires to join forces with Verstael Besithia to bring down the Wall around the Crown City so Niflheim may invade.
Founder's Day is a day Insomnia celebrates the founding of their city. It is in the honor of the Founder King, Somnus Lucis Caelum.
Testing, 1-2-3. Is this thing on? Greetings, people of the Kingdom of Lucis! Do forgive me for interrupting the festivities… …but I must tell you this day of rapturous revelry shall be your last. Call it "divine retribution." False kings and fraudulent nations are fated to perish. Sins of the past must not go unpunished. The time of reckoning is of hand! [And today, you shall atone on behalf of your forebears. Resist, if you wish, but know that your fight against fate is a foolish and futile one.]The Adagium's ultimatum broadcast during the Founder King Day ceremonies
Ardyn daemonified a border patrol guard, Mars Sapientia, assuming his identity to access the city during the Founder's Day festivities. Upon reaching the Citadel, he attempted to summon Ifrit to raze it. Due to Ifrit's fickle nature, the Astral did not immediately arrive, and emerged in a separate area of the Crown City. Nevertheless, Ardyn managed to decimate large portions of the city, and hijack the broadcasting network to announce that the kingdom was fated to fall due to being a "fraudulent nation". The reigning king, Regis Lucis Caelum, vowed to personally confront him. Clarus Amicitia commanded the Royal Guards, including the Crownsguard and special forces, to fight off Ardyn and Ifrit, as well as evacuate the civilians, with the Vanguard being dispatched to deal with Ifrit specifically. Eventually, they identify Ifrit as an Astral, although at the cost of most of their forces.
After wiping out a significant number of the Royal Guards, Verstael Besithia told Ardyn to destroy the devices that amplify the Wall. Ardyn located three of them, also fighting various guardians modeled after the Rogue and the Fierce. Clarus Amicitia deduced that Ardyn was connected to the empire, and ordered the Lucian forces to target him as well as reinforce the amplifiers. While fighting the guardians, Ardyn realized that the people of Lucis had the Kings of Yore protect them during times of crisis, and deduced he could exploit that to bring out Somnus to settle their score.
After disabling three Wall amplifiers the Wall yet held against Besithia's estimations, who ordered Ardyn to withdraw. Ardyn refused, intending to kill King Regis, shocking Besithia as such had not been part of the plan. He confronted Regis at the Citadel, unveiling his royal arms, causing Regis to realize the attacker was the infamous ancient criminal, Adagium. Ardyn bested Regis in combat, demanding that he summon the Old Wall. Regis was shocked Ardyn could even know about the Old Wall, but Ardyn explained he wanted to meet the Founder King. He angrily demanded for Somnus to come out, threatening to kill Regis.
After Regis was knocked out cold Somnus arrived, declaring that Ardyn needs to be sealed away again. Somnus gradually admitted that he had been jealous of his brother, though still insisted that the throne was not fit for a sinner like Ardyn. After Ardyn finished off Somnus's Old Wall form, Somnus admitted that what he did was unforgivable, though he ultimately did so for the welfare of Lucis, and asked for Ardyn's understanding. Ardyn was infuriated that he would ask for that after taking everything from him. Somnus faded away, vowing to pray for the repose of Ardyn's soul.
Ardyn failed to weaken the Wall enough to breach the city, with Niflheim thus failing to take over Lucis. As such, the empire was forced to assume another strategy, which would prove successful with the Fall of Insomnia years later.