The Gaderene were a parasitic species.
By the 20th century, their homeworld was dying. During World War II, two Gaderene embryos were transported to Earth and landed near Culverton to prepare the way for an invasion. However, the matter transmitter that was used became unstable. One of the young Gaderene, Bliss, was stranded, unable to communicate with her home world. She took a human as a host to survive. The other embryo was mutated during the transfer and hibernated in the marsh near Culverton.
Thirty years later, the Master helped Bliss reconnect with her people. He wanted to help them invade Earth mostly to watch them destroy humanity. But when the matter transmitter was finally repaired, Bliss declared the Master no longer useful and tried to kill him. Instead, he killed her with his Tissue Compression Eliminator. Due to the mental connection between the Gaderene, this stopped the invasion. (PROSE: Last of the Gaderene)