how do you build outposts in the Umbral realm?

[Witch ~]

Is it even possible?

[Terrkas]

There are beacons of light. For example beneath the infestation. Those enable outposts.

[Astronimo]

Destroy infestation and it gives a beacon of light on it's place. You can build the outpost there (and expand 2 times).

[utilityguy]

You can only Build outpost of light on terrain that contains that giant concrete circle slab, which may by hiding under infestations.