The Prethoryn Scourge are an extragalactic race of unclassifiable organisms.
The Prethoryn scourge have many different forms but most species deal with the "helmsman" form, a form that consist of a large eyeball planted on a stalk with prehensile tentacles. This form seems to be the only one that contains intelligence and language albeit untranslatable. The one for sure way to initiate dialogue with a Prethoryn helmsman is to have some kind of psychic ability. If successful, the helmsman would explain that they are running from something called the Hunters.
It is unclear what type of planet they evolve from nor what galaxy they hail from but one thing is clear: they invade galaxies to feed. They first probe the galaxies that they want to invade by sending in vanguard fleets to cause as much destruction as possible. By breaking the will of their would-be food they then send in their larger fleets including their queens to mop up any form of resistance.