Cepheids are a sentient species resembling spheres of black rippling water capable of hovering, changing shape, expanding, and rushing with dangerous force. They are able to extend their hover ability to other beings, usually involuntarily. How they manage to pilot their boomerang-shaped spacecraft is a mystery but it is hypothesized that in large numbers they can cause those to hover as well.
Cepheids reproduce by pouring small drops of themselves into the orifices of other sapient beings, usually while they are asleep. Infected beings feel fine for a few days and then usually begin feeling weakness, nausea, and severe chills. Then their eyes sink into their faces and they "melt" into a black waterblob. It's not fun to watch and probably no joyride to experience either.
It is hypothesized that the Cepheids were created by a rogue scientist on Reticulon I, but no one really knows because even if that's the case he's long gone now. Suffice it to say that they decimated the Reticulon Grays, beginning a war. The Grays chased Cepheids around the galaxy, freezing as many as possible to keep in suspended animation and trying to identify and innoculate victims before they transformed. Currently there is no way to cure someone once they have been a Cepheid for more than a few hours, but scientists are diligently trying to discover one and have reported some promising breakthroughs.
Cepheids infected an Earthling named Kelly Taylor and later her brother Morgan. Shortly afterwards their ship was destroyed in a massive explosion caused by the other species deliberately overloading Element 115 in their own ship.