A gaseous alien parasite that feeds on orgasmic energy. Composed of vorax and ceranium gases, Earth's atmosphere is poisonous to the alien, so it needs to take a human host to survive for prolonged periods. It vies for control with its host, causing physiological changes that eventually cause the host's internal organs to explode.
The alien also makes its host secrete a blend of ultra-powerful pheromones that cause tremendous sexual attraction in those around it for the purposes of feeding. Coupling with the host is fatal, causing the host's partner to disintegrate into a pile of dust at climax and allowing the alien to absorb the energy from the orgasm.
A specimen came to Earth in a meteorite in the early 21st century and landed near Cardif. Although the technological level of the entitiy is unknown, it refered to the meteorite as his ship and said it came to Earth by design, so it has some knowledge of interstellar scale and has the ability to manoeuver through space in a container of some sort.
Source: Torchwood "Day One"