The Tsol'teth are the descendants of the Tsol'aa warriors, known as Qui'anar, who fought during the battla at Nishnatoba. Upon returning home, these Tsol'aa began to disagree with others of their race in regards to how to treat other races, (considering these races inferior, and worthy only of enslavement.) While it is possible the effects of battle may have hardened the Qui'anar, other factors may have been at work, though no solid evidence has been found for any current theories as to what may have caused this change.
The Tsol'teth left their cousins in an exodus that eventually led them far below the earth. There they enslaved other races; goblins, hobgoblins, ogres, and an insect-like race that may have been horkvali; to serve them and worship them as gods. During the travels of Nicator, in his search for Seleucar, the Tsol'teth took an interest in the surface world once more. Three Tsol'teth in particular were involved in the war to come: Agith'maal, Terrin'ukia, and Gattan'lier. The strongest amongst them being Agith'maal. These Tsol'teth summoned forth Pazuzu in hopes of learning how they might better achieve their new goal, the destruction of all mortals who roamed the surface world, though the demon lord's simple answer, "Seleucar." frustrated them.
The Tsol'teth were eventually able to divine the meaning of this word, but were unable to travel to the surface world with their army of slaves, as the sunlight burned them. They prepared for some time, and sent curses and plagues to weaken Nicator and his followers, while amassing a great army and training them to better understand the surface world. Eventually the Tsol'teth slew their slaves in a great ritual. The slaves were then ressurected, but with cold hearts and an immunity to the harsh light of the sun. Only their general, Matic Ridley, was spared, trained in the ways of the surface world and used by his masters to help prepare the new army for battle. Matic Ridley became vengeful, however, feeling the true soul of his people had been lost.
Each of the three Tsol'teth took their forces to different major cities of the realms. Agith'maal to Seleucar, Terrin'ukia to Ashtan, and Gattan'lier to Shallam. They nearly decimated these cities until Agith'maal was betrayed by Matic Ridley, who pulled away the cloak of darkness protecting the Tsol'teth lord and stabbed him in the back. Matic Ridley aided the Seleucarians, and they were able to defeat Agith'maal and his forces by tearing out the Tsol'teth's heart. An unforetunate side effect of this was that the occultist Adchachel devoured the heart to gain essence, and later went mad.
Seleucar was able to aid Ashtan and Shallam after the defeat of Agith'maal, defeating the other Tsol'teth, though not without many casualties.
No more was heard of the Tsol'teth, until the modern age, when an unknown Tsol'teth invaded the previously hidden Barony of Dun Valley, slaying its inhabitants with the aid of ogre and orc slaves. Before his plans could be realised, however, he was betrayed by one of his slaves, Vukub Ysin’zhu. An apprentice of the Tsol'teth, Zh'risia, survived, and managed to manipulate the orc, Kroul. Kroul was under the command of Vukub, and through him Zh'risia nearly succeeded in his vile plans, the extent of which are unknown. Eventually, an army of Achaeans, grouped from many walks of life and united in their purpose to stop the machinations of Vukub, invaded the fortress and found Zh'risia there, ruining his plans and slaying him after a great battle.