Attack during Founder's Day

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, true identity Ardyn Lucis Caelum, infiltrates the Kingdom of Lucis during the Founder's Day festival on M.E.734 in order to burn it to the ground as revenge for the treachery of its founder, his brother Somnus Lucis Caelum.

Prelude
Driven mad from reliving his betrayal at the hands of his brother Somnus Lucis Caelum millennia prior upon daemonifying the Astral Ifrit, Ardyn Izunia, promoted to Chancellor of the Niflheim Empire for revolutionizing the empire's military, proceeded to spread the Starscourge while infiltrating Lucian bases and daemonifying the inhabitants to learn more about the kingdom, twisting him into a wicked echo of his former self in the process.

The attack
Eventually, after finding an opportunity to strike back at the Kingdom of Lucis, he proceeded to daemonify a border patrol guard, Mars Sapientia, before dawn, then assumed his identity to easily access the city during the Founder's Day festivities.

After entering the city premises via taxi, he then disembarked while claiming he was on "special assignment", and upon reaching the immediate premises of the Citadel, then attempted to summon Ifrit to burn the Citadel down. However, due to Ifrit's fickle nature, Ifrit did not immediately arrive, and emerged in a separate area of the crown city. Nevertheless, Ardyn managed to raze large portions of the city, and proceeded to hijack the broadcasting network to reveal that the Kingdom of Lucis was fated to fall due to being a "fraudulent nation". The reigning king, Regis Lucis Caelum, received the message, and while unaware of his identity, nevertheless realized he had ill intentions for the city, and vows to personally confront him. Clarus Amicita commanded the Royal Guards, including the Crownsguard and special forces, to fight off the two attackers as well as evacuating the civilians, with the Vanguard being dispatched to deal with Ifrit specifically. Eventually, they managed to identify Ifrit, although at the cost of most of their forces.

After wiping out a significant number of royal guards, Verstael Besithia contacted Ardyn explaining that a disturbance occurred, and after failing to get Ardyn to explain himself, told him to destroy the devices to amplify the wall. Ardyn then located three of them, also fighting various guardians modeled after the Rogue and the Fierce. After one of the wall amplifiers was disabled, Clarus Amicita deduced that Ardyn was likely connected to the Niflheim Empire and disabling the wall amplifiers for Niflheim, and ordered them 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 the score. After disabling three wall amplifiers, Besithia attempted to get Ardyn to withdraw, though Ardyn refused, citing he intends to kill Regis Lucis Caelum, shocking Besithia as such had not been part of the plan. He then confronted Regis as the latter emerged from the Citadel, and after bidding him greetings, revealed he came for the king before unveiling the royal arms, causing Regis to realize the attacker was none other than the infamous ancient criminal Adagium. Regis then attempted to stop Ardyn, though Ardyn ultimately bested him in combat, before demanding that he summon the Old Wall. Regis expressed shock that Ardyn could even know about the Old Wall, before Ardyn started kicking him explaining he wants to meet the Founder King to settle a score with him. He then angrily demands for Somnus to come out, threatening to torture Regis if he didn't immediately arrive. However, he ended up knocking Regis out cold instead, to the extent that Ardyn assumed he may have accidentally killed him. Nevertheless, Somnus heard his challenge and arrived to deal with Ardyn, citing that Ardyn needs to be sealed back, although Ardyn refused. Eventually, Somnus gradually admitted that he did feel jealous of his brother, though still insisted that the throne was not fit for a sinner like Ardyn. Ardyn alongside Ifrit then managed to finish off Somnus, though Somnus then admitted what he did was unforgivable, though he ultimately did so for the welfare of Lucis, and asked for his understanding. Ardyn, however, was infuriated that he would ask for that after taking everything away from him. Somnus then faded away, although not before vowing to pray for the repose of his soul.

Ardyn, still infuriated with his brother, then attempted to strike down Regis in cold blood, although Bahamut disabled Ifrit and teleported Ardyn to his realm.

Aftermath
Ultimately, due to Ardyn's going AWOL to attack Regis, they failed to destroy all the necessary wall amplifiers to weaken the wall enough to breach the city, with Niflheim failing to take over Lucis. As such, they were forced to conduct another strategy, which ultimately would prove successful with the Fall of Insomnia years later.