Agith'maal, also known as Master Blue Dark in common Achaean, was a warped Tsol'teth who resided in "Anzari-tarin" - the World of Darkness; an underground realm. He commanded an army to follow him up into "Tezlari-tarin" - The World of Light (the Prime material plane), where they initiated the War of the Deeps against Seleucar, Ashtan, Shallam, and just the people of Sapience in general.
He was slain by Matic Ridley, a defected hobgoblin.