Aszachaeth was created as a favored agent of the Overmind during the First Great War, and as such, was endowed great power from his creator over other members of the Swarm, however, upon the Overmind's death, he not only regained his free will, but developed these abilities further, gaining a mental immunity to domination by the means of cosmic psionic powers.
By the end of the Brood War, the Wise Old Torrasque had come to lead a group of Zerg mercenaries known as the Swarm of the Torrasque. At one point in time, it offered its services to the Terran Dominion to deal with the New Trinidad Pirates, however, this alliance proved fruitless when the Dominion tracked down Aszachaeth and held him on a secret base located on the moon of Demeter, attempting to find a way to take control of his Swarm.
Using stolen Jal'Ras based prototype psi-dampening technology, they were able to subdue his psionic emanations to a minimal, weakening him to the extent that he could not break free of his psionic prison. During his captivity, many agents of his swarm came to save him of their own volition, but were captured and experimented on until their deaths. All experiments proved unsuccessful however, and the duration of his capture was not unlimited, thanks to the efforts of 3 Ghosts mercenary operatives hired to find out what the Dominion was up to, copy the data on the project, and then destroy the original files.
Kassandar, a highly respected Grandmaster and High Executor of the Protoss, was also drawn to the world due to the powerful psionic presence of The Wise Old Torrasque that even the Psi-Dampeners could not completely mask. He covertly aided the Ghost mercenaries in their mission and ended up speaking to Aszachaeth for a brief moment, however the power was cut from a stray bullet, giving Aszachaeth just long enough to escape before the backup generators kicked in.