Sex Gas
Sex Gas is a type of intelligent gas, one of which came to Earth in an meteor which fell near Cardiff (presumbably attracted by the Cardiff rift). The gas seems to have an addiction to the energy produced during heterosexual orgasm. The gas on Earth found that the energy produced during sex was the best. This sex is fatal for the other participant, since they will be disintegrated and absorbed at the climax.
In its natural form, the sex gas is a thick purple gas with high quantities of vorax and ceranium. It can't survive in the atmosphere of Earth for very long and will quickly turn into a pink powder.
While inside the human, the sex gas causes a variety of changes, both mental and physical. The host gains an enormous sex drive,sharing in the gas's addiction. Despite its need to absorb the energy produced by heterosexual sex, it had no objection to homosexual sex.The host will develop special pheromones which are irresistible for other members of the species of either gender. Internally, the heart and brain will increase in size and the lungs will shrink, which will eventually explode, killing the host. (TW: Day One)
