The Immortals were created to rule Earth in the wake of the confederation of the planet. The Immortals were then chosen to pilot the 100 odd ships that made it off Earth, and stayed awake throughout the 1000 year journey to Jove. They then went to war with the Gods, resulting in their decimation and the loss of their superiority on Jove. In the events of the Prince of Ruin, he, and the Blood Princess are the last (known) Immortals, although there is rumoured to be Immortals hidden in the planet.
The Immortals are resistant to all disease, are highly intelligent, have undergone thorough genetic modification, have undergone thorough training, do not breathe, are programmed not to be able to kill themselves, and can do some odd things (possible prescience? possible telepathy?) that humans cannot do, however only the original Immortals have the last feature. They also never die through aging or genetic conditions.