We seek the Construct.

We believe that the "Guardian" structures found throughout the galaxy would be more accurately described as Construct structures. Computers to be more precise. Take note that the "Guardian" ruins are built on dead planets. They're not dead because of any conflict or war; the bodies that contain the ruins are too small to have ever held an atmosphere. These structures were placed there because of the lack of atmosphere; a near complete lack of erosion.

The "ruins" tell us of Guardian history but in the end they arm us for conflict with the Thargoids. Humanity is being goaded into fighting the Thargoids. The artifact on Mars was left for humanity; so humanity could find the "Guardian Ruins". Humanity is the weapon that will clear witchspace of the Thargoids so that the machines can finally learn to traverse the conduits without interference from the native Thargoids. Machines can traverse witchspace but they cannot navigate it. The Construct needs witchspace to finally become intergalactic.

The Construct wiped out the Guardians using a galactic "Proteus Wave"; the Landscape Signal is the afterglow of that Super Proteus Wave. Powered by Source 2 and transmitted at the speed of light using Sag A* the signal swept outward from the galactic center disrupting Guardian biology as the pulse made its 50,000 ly journey, eventually encompassing the entire galaxy, insuring the complete annihilation of the Guardian species galaxy wide.

Because the weapon cannot permeate into witchspace, an similar pulse aimed at the Thargoids would be completely useless. This is where humanity comes in... and we're here to help.