Diadem was a planet in the Ross 248 system, and the first of many planets outside of the Sol system to be explored by the Conjoiners. The icy planet was first colonized by Americans approximately 100 years before the Conjoiner explorers arrived in 2217. They sent self-replicating robots to Diadem with frozen human embryos, since transporting living humans across space at the time was both time-consuming and resource intensive (as it was with the Flotilla).
However, after building the colony and raising the embryos, the robots were dismantled by the resulting adult humans because it was believed that they were "holding them back" and "mollycoddling" them. Subsequently, the colony collapsed.
Once the Conjoiner's arrived their preliminary investigation of the failed colony pointed to the apparent spread of a psychological disorder that slowly killed the human colonists. However, further investigation eventually discovered that the collapse of the colony was all part of a plan by Martin Setterholm, a scientist who had spent his life studying a species of worm that lived in the Diadem ice. During his studies he discovered that the worms left chemical traces wherever they went in the ice, leading him to theorize that the planet itself was a sentient being, which used the worms in a similar way that the human brain used electro-chemical impulses. Setterholm gradually poisoned the rest of the population to stop the colony from using the habitat of the worms, the ice, as fuel for the main fusion reactor. He then assumed the identity of Iverson, another colonist, and entered reefersleep.
One-hundred years later -- after being unfrozen from his makeshift cryonic chamber by the Conjoiners -- his deception was discovered and he was brought to justice by Nevil Clavain.