Of all the alien races encountered by civilized beings in the Milky Way galaxy, none are more feared than the Ylir. Frightfully intelligent, powerful and utterly ruthless, the Ylir exist only to devour other species, wiping them from the galaxy and using their planets to fuel their unholy mission of galactic extermination.
No one knows where the Ylir came from other than their origins being somewhere towards the center of the galaxy. It is speculated that the higher radiation mutated a benign species into the rapacious monsters that would become known as the Ylir, others propsed the idea the Ylir had been genetically engineered shock troops for an advanced alien race that then turned on their masters. Whatever their origins the Ylir aren't talking and expeditions along their path of destruction have only revealed planets devoid of life, brutally stripped mined of resources to fuel their mad crusade.
A Ylir "murderswarm consists of hundreds of thousands of the race descending upon an unsuspecting planet in troop carriers, bursting out and murdering anything and everything in sight while the ship obliterates everything in orbit and then bombards the planet below. The Ylir do not care about heavy losses as they breed quickly.
It was thought all the Ylir had been destroyed centuries ago, but a murderswarm attacked Earth, it was suggested that the Ylir were either drawn there or somehow guided in that direction. A mother ship was about to unleash a hoarde of drop ships on the planet but Earth's officials were forewarned by the Office of External Relations and a desperate plan was formed to use all the ships of the ECLIPSE fleet. The Ylir was not expecting such resistance from a "primitive" planet but they destroyed almost all the vessels easily. Almost all; the last one contained several hydrogen bombs and flew right into the mother ship, obliterating it. Hundreds of brave men and women died and the result was the Ylir never sent another murderswarm to Earth because they had no idea how the last one was defeated. Whether this is the last anyone will have seen of the Ylir is anyone's guess.