The Ones were a threat revealed to Xar Kerensky by his and Zalaria's future son Derek, claiming that in the future they invaded the galaxy and were a far greater threat than even the Altarin'Dakor. In the future, he said, the Ones had already taken over the entire galaxy, and were on course to consume the entire universe. They were so powerful that even future Derek could only take on several of them at one time. He also said that they were led by Altima, who had been the supreme Warlord of the Altarin'Dakor. All of this was related to an ancient evil Kajeat Traveler named Malduke, though Derek did not know much more than that. He was looking for Malduke, but had had no success, which was one reason he saved his father's life during the Battle of Varnus.
Little other information was known about the Ones. The Travelers may have known about them, since they were refitting their starfleets when Icis Novitaar went on trial on Kajarn so that their fleets would no longer use organic technology, speaking of a coming threat. The Crinn led a mass exodus from their world into unknown areas, claiming they were "fleeing the coming darkness". There was also a rumor that a Jedi used the Scepter of Karanishma to look far into the future and saw the Ones, and that he died immediately.
Xar Kerensky and Icis Novitaar hoped that the Travelers might be able to reveal more information about the Ones. But as Icis had already been permanently banned from his homeworld by that point, he did not know how they would be able to find the assistance they needed.
It was later revealed by Angol Moa that the Ones were actually the spirits of the beings left behind in the Travelers' home dimension, and that the Entity was actually the collective consciousness of all of the Ones combined. New Ka'jeat were brought into the world during the mating process that drew forth a One from their home dimension and "cleansed" it, bonding it with a new host to give it sanity. The Ones' all-consuming desire was to break into other dimensions and consume all life there as well. Since the Ones were the Entity, and the Entity controlled Altima (and by extension the Altarin'Dakor), it was realized that these threats were all one and the same, and would have to be dealt with. Also, Angol Moa acknowledged that she was technically responsible for the creation of the Entity, the Ones, and by extension the Altarin'Dakor, as well.
Angol Moa revealed that Altima, supreme leader of the Altarin'Dakor, had in fact no interest in conquering galaxies but rather desired to release the Ones into the universe to consume all life. In order to do this he needed to use the powers of Maldule, a Ka'jeat Death Child, who was actually a partially-redeemed One. Using Malduke's spirit as a kind of bridge, Altima could unleash the Ones into the world en masse.
Individual Ones were so powerful that no being alive could stop one. Only Derek, son of Xar Kerensky and Zalaria, could destroy one, and even he could only take on a few at a time. The quadrillions of Ones waiting to be released were strong enough to consume all life in the universe - each would "possess", or absord, a living being and take on its flesh, while continuing to spread its insatiable desire to feed on all life around itself.
Discovering a way to stop the Ones from entering the universe was eventually revealed to be the ultimate purpose and goal of the New Imperium plot line.