Xenomorphs are incredibly dangerous creatures of unknown origin. Their physical makeup has been honed to perfection, so that they are almost unkillable and extremely parasitic. They have the underlying goal of spreading their seed to every living thing in the Galaxy, a bit like some other people we already know.
The Xenoporphs are a hive mind, ruled by a Queen, who spread their infection through smaller 'Facehuggers' that latch onto the faces of their victims and implant a small growth into them. The Facehugger then dies and the victim wakes with no visible side effects. However, the growth soon develops into a new life form by extracting DNA from the host and feeding off them.
The newborn Xenomorphs then burst out of their host's chest cavity in the form of 'Chesbursters' and begin to grow. They grow very quickly, and their appearance and traits are based on the characteristics of whatever life form it spawned from. The victim then dies and the Chestburster sheds its skin and grows into a fully mature Xenomorph.
The Xenomorphs have no known homeworld and little or no technology, they travel around the Galaxy by stowing away on board freighters and scout ships. Over the years, the League has learnt how to deal with minor infestations but occasionally entire ships can succumb to Xenomorphs infestations. This is very dangerous as a thriving colony of Xenomorphs crashing on an inhabited planet could result in the loss of the world. The League is currently trying to find a cure for the Xenomorph infection. The Yautja seem to hunt the Xenomorphs for sport but even they are succesptible to Facehugger implantation, and the Xenomorphs that spawn from Yautja are far more powerful than the Human-incubated ones.
The spread of the Xenomorph infestation has been largely ignored by the League Council, with many reports of Xenomorph attacks passed off as nonsense. However, for the sake of precaution, there is a footnote in the League Protocols that dictate that any ship of any kind with a reported Xenomorph attack is to self-destruct immidietly with all surviving crewmembers put into a 40-day quarantine. However, League Xenobiologists claim that these precautions are still not enough. As the debate continues, as does the Xenomorphs genocidal campaign to spread across the Galaxy.