Ancient Wyrm turned Dracolich, former attempt at Assuming Tiamat's throne.
The twin God-Throne of Bahamut-Tiamat can be assumed through great effort. Zeiram was one such would be usurper.
Empowered by the weakening Bahamut, a Draconic Ascendant of the Feywild, the heroes crept through the pocket realm of Zeiram into the fortress and pin of the realm. Releasing 4 unknown powers pulled the pin on that realm, unleashing the dragon souls Zeiram had imprisoned and preserved to achieve his ascendancy.
Centuries of work undone, Zeiram seeks now to conquer the long and hard way, with his undead army and a lot of death.
Naturally, his defeat has colored his disposition, such that while conquering is his primary aim, he now additionally carries powerful desire for revenge.