Unmanned suicide weapons, the Unknowns were responsible for devastating the area during the events prior to Last Raven.
Originally seen in Armored Core: Nexus, they appeared in the underground ancient structure after the control units were destroyed by the Raven. They swarmed to the surface and attacked everything on the surface, causing irreparable damage to the cities, structures, and the assets of all known organizations.
In the wake of their attack, the corporations fused to form Alliance to help rebuild the world. In this, they made an enemy in Jack-O who formed Vertex. Their current fate is unknown as they appeared in several missions (three to be precise) but with the destruction of the Internecine, it can be presumed that they, along with the Pulverizers, were gone for good.
They appear to be about the size of a large missile, and simply attack a target by swarming it and then exploding upon impact. Unlike normal missiles, they show the capability to change direction instantaneously. Also, while doing moderate damage, they are capable of huge stun, meaning that one hit will stop an AC in its tracks and leave it vulnerable to a volley of these weapons. It is possible to lock onto these weapons but due to their trajectories, they can be hard to near impossible to lock on to and this makes fighting them (along with their massive numbers) extremely difficult; evasion is your best option.