Skel'Ske was an experimental Hith spacecraft. The name was the Hith word for the sound made by the wings of JakkatKajjat, the twin-headed Goddess of Justified Retribution, as she passed overhead at dawn, tearing the souls of her enemies from their bodies.
The Skel'Ske was powered by icarons, which were not dangerous to the non-carbon-based Hith. They developed it during their war with the Earth Empire. After they lost, the ship was taken to Earth, where Tobias Vaughn had it examined by INITEC to learn if the technology could help defend the planet.
The ship naturally rested in hyperspace due to its icaron-powered engines. When the ship was accessed from normal space, the opening released icarons which cause insanity in the Overcity it was in. The Seventh Doctor was able to find and destroy it, stopping the madness from spreading further. (PROSE: Original Sin)