The Principality of Letidius was an ancient kingdom.
Around 500 AK(?), when the kingdom was at the height of its prosperity and glory, the king offered a massive reward to anyone who could grant him immortality. A massive wave of violence broke out across the land as sorcerers competed for the reward -- in the words of Lina Inverse, "I mean, say you do create your own Potion of Immortality, how are you going to test it? Exactly. Those experiments always end in tragedy."  A mere two years after the reward was announced, the king was dead and his kingdom was destroyed by civil war and foreign invasion. Sorcerers refer to the period as the Dark Age.
While most of the knowledge and treasures that were amassed in Letidius during the country's era of prosperity were lost, some fragments are known to have survived. In particular, the high priest of the royal court escaped to Raizeel with a Claire Bible manuscript that would later be used to create the Demon Beast Zanaffar.