Bliss was a Gaderene.
Bliss and another Gadarene were sent to Earth as embryos during World War II to prepare the way for an invasion. However, the matter transmitter that was used became unstable. Bliss was stranded, unable to communicate with her home world, and 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. (PROSE: Last of the Gaderene)