The Thul are a slug-like race that served as minions to the Il'Ri'Kamm Hive Mind, inventing the original OtherSpace Drive on the Hivers' behalf. First encountered by Sanctuary in Hiverspace, these aliens abducted refugees from the colony vessel to conduct experiments.
By the time Sanctuary encountered the Thul, only a handful of the giant slug race were still alive. Due either to their small numbers or to flaws (either deliberate or intentional) in their own Hiver-inspired evolution, the Thul were unable to reproduce effectively. Initially they kidnapped young humanoids from the colony vessel in the hopes that they could use pituitary fluid, which was found in the highest concentrations in adolescents, to synthesize a reproductive method of their own. Ultimately it is believed these experiments were unsuccessful.
When Sanctuary was seeking a means to return to Normalspace, it sent several expeditions in search of the Thul. Ultimately, it was on Comorro Station that members of the colony vessel found the means to track down the leader of the Thul, Ancient Shog.
Shog was later located on the planet Nemonus, trapped(?) in a swamp. It demanded transportation off the planet before it would aid Sanctuary. Ultimately, a group from the cargo vessel Asylum that included John Christian Falkenberg and Remy LeBeau located Shog and brought it to Sanctuary.
There, the giant slug demanded that it be given frozen humanoid bodies in exchange for helping the ship return home. It was its intention to use these bodies to further its efforts to create a reproduction method for the Thul. At the suggestion of Falkenberg, the Sanctuary Senate gave Shog the frozen (but still alive) bodies of Lord Fagin and Dimitri Volstov. Shog then gave Sanctuary the means to return home.
The Thul performed experiments on their two frozen humans, transforming Fagin into a floating brain that remained alive and aware in a tank, and Volstov into a gruesome half-human half-Thul creature named Voshtov, who became Shog's underling.
When the Thul's creators and benefactors, the Hivers, repented of their previous hatred for corporeal life and refused to aid the Thul in further attempts to reproduce, the Thul transferred their alliegance to the Hivers' mortal enemies, the Kamir, who promised to help the Thul perpetuate their species. In this way, the Thul became allied with Grim, who was resurrected by the Kamir on Tomin Kora to serve as their agent of vengeance.
With the aid of Fagin and Voshtov (and likely Grim as well) the Thul were able to create the Thul Resequencing Virus or TRV by modifying a strain of the Legion Virus. This virus would transform humanoids into Thul-ish creatures like Voshtov, giving the Thul a means, however imperfect, of reproduction. Also, working with Grim, the Thul stole a Centauran invention, the Moebius Device, which they attempted to use to alter history so that Sanctuary never escaped the Kretonian invasion. This precipitated the Moebius Effect Crisis, which restored an alternate Earth, Luna and Mars which was ruled by Colin Neidermeyer.
Neidermeyer also appeared to have been in cahoots with the Thul, who created for him the Crystal Cathedral, a secure, cloaked space station from which he could continue to control Sol System while under assault from outside forces. With Neidermeyer's aid, the Thul were able to infect most of the population of Luna with the TRV before a cure was found, foiling the Thul's last attempts to reproduce.
At the conclusion of the Moebius Effect Crisis, it is believed that Ancient Shog himself was killed, leaving Voshtov as the sole surviving Thul, in possession of the Thul's vast technology. It is further believed that Voshtov found the means to reverse his own transformation, becoming Dimitri Volstov once again, and rendering the Thul effectively exinct.