Let's see... It was founded in the land of Gyr Abania in the 5th astral era. The Great Flood of the 6th Umbral Calamity swept over the land submerging the city. Over 1,000 years later Ala Mhigo would be built over the forgotten city, but the last king of Ala Mhigo, Theodoric. He grew paranoid towards the end of his reign, he had his own family members imprisoned on invented charges, and were transformed by the court thaumaturges to guard the riches hidden below. But because he didn't trust his own countrymen, he enlisted the help of foreign thaumaturges. The final boss is the mad king's cousin who had a strong sense of justice and was known for trying to council the king against his decisions, and for this he was punished for "treason".

Basically all that you already said, but this is all the page in the Lorebook had to say.