The Vasteryoi were a race whose home worlds had been invaded and colonised by humans.
A group of Vasteryoi and Tendragons invaded the SORDIDE Delta space station, using a sickness which heightened emotions to spearhead their attack, in an attempt to take Shadow-Space from the humans as the humans had stolen their worlds and exploit it for their own empire's space travel pursuits. However, on arrival they discovered that the surviving crew had retreated to Shadow-Space, having discovered a means of curing themselves. Analysing their cure, the invaders devised a new plan, in which they would reinfect the now immune crew members and send them out as plague carriers, wiping out the human colonies.
Pretending to be Maschera, guardians of the shadow dimension, they attempted to extract the humans from Shadow-Space, but were frustrated when the Fifth Doctor, Nyssa and Hannah Bartholomew were drawn to the station and forced into Shadow-Space. When Nyssa returned to the home dimension they tried to kill her, as she was an unwanted alien who would not function as a plague carrier. All of the remaining Maschera were killed when Shadow-Space was destroyed by the Dead Man. (AUDIO: Masquerade)