For those who have not completed Xenoblade Chronicles 2, this page may contain spoilers regarding the plot. Discretion is advised.
The Kingdom of Torna was destroyed in the Aegis War; when Addam, the hero of Torna, did not return to the Tornans' new home of Genbu, there was mass confusion about who would lead their people. Those who followed Addam believed that humans, Blades, and Titans should live together, while his rivals believed that humans were the masters, and that Blades and Titans were their tools. Amidst the confusion, the Tantal family claimed to carry the bloodline of Addam, in order to bring stability to the new nation, and formed the Tantalese royal dynasty, using Addam's name to win over the people. King Eulogimenos is descended from this line.
King Eulogimenos' son, Prince Ozychlyrus, left the borders of Tantal against his father's wishes to travel the world when he was a teenager, causing him to be banished. Eulogimenos maintained the isolationist policies of the nation until word reached them that the Aegis had awakened. Praetor Amalthus later sends Rex as an envoy to Tantal with a special message.
When the party arrives at the Kingdom of Tantal, Eulogimenos reads the message and tears it up in frustration. He then captures Pyra, intending to use the ether accelerator to destroy her. He believes this will keep the world safe; however, the party is able to alter the trajectory of the weapon and save Pyra with the help of Zeke.
After Rex is defeated, Eulogimenos reveals that the Tantalese royal family is not, in fact, descended from Addam as they had claimed; they saw an opportunity for power and took it, believing it to be the best course of action for their nation. He reveals that Indol agreed to keep this secret in exchange for a regular supply of Core Chips. In the letter, Amalthus demanded that, in addition to the Core Chips, Eulogimenos hand over the Omega Fetter or risk having their secret exposed. This led to him to try to destroy the Aegis in order to reduce Indol's leverage on them.
King Eulogimenos also reveals that in a Tantalese book recording Addam's heroics, a third Aegis Sword is mentioned. The sword was too powerful for Addam to handle, so he reportedly sealed it away in an undisclosed location, and Azurda reveals that he knows the location. Before departing, Eulogimenos asks Zeke to protect Rex and to make sure his passion never dies out.
When the Artifices attacked Alrest under Malos' control, Eulogimenos is seen escorting people out of Theosoir and to safety.