The Thanatos, also known as the Worldbane, was a machine regarded as the greatest super weapon ever created.
The Thanatos was a weapon capable of destroying whole planets. It could probe people with tentacles that interfaced with the individual on a mental and physical level. A judgement engine inside the machine decided whether a race was worthy of survival, based on their potential to evolve. It produced the Inuk to serve as its antibodies and was intelligent enough to herd enemies into traps. (COMIC: The Betrothal of Sontar)
The Sontaran Colonel Snathe searched for the weapon, eager to use it to destroy the Rutans and return glory to his name. He was looking for it on the planet Serac with the Betrothal of Sontar. After encountering the Tenth Doctor, he claimed he would help the Sontarans find the machine.
They found the Thanatos under the ice, where it was guarded by the transformed Inuk. They destroyed Snathe's aircraft, but the Doctor reprogrammed a drone and used it to escape — they believed. They were just being chased deeper in the Thanatos. Rose Tyler and Lerox caught up with the Doctor and Snathe, just when Snathe began to interface with the Thanatos.
Unfortunately, Thanatos judged that, due to generations of cloning themselves instead of evolving naturally, the Sontarans were a stagnant race and Sontar needed to be destroyed. Lerox was shown to be capable of self-sacrifice, so the Sontarans were spared, but Snathe was captured by the Inuk and ejected outside during the middle of a Silverfire, killing him. (COMIC: The Betrothal of Sontar)