Nabonidus was the last king of the Neo-Babylonian Empire, reigning from 556-539 BC.

Nabonidus was the last king of the Neo-Babylonian Empire who, a few years into his reign, disappeared into the Arabian oasis of Tayma, returning years later and just in time to lead his armies against the ascendant forces of Persia under Cyrus the Great. Why he abandoned the city he was ruling over, and what happened in the lost years of Nabonidus remain a mystery.

Source: The Lost Years of Nabonidus, Last King of the Neo-Babylonian Empire | Ancient Origins