I was always fascinated about those looming black basalt cities of the Flying Polyps. I came across the wikipedia page of these Lovecraftian Monsters, with some mediocre art. I am sure I can do better.

The basic premise is that these spectral monstrosities infest planets. They are truly alien, far more alien than most Lovecraftian monsters and communication or negotiation with them is more or less impossible. The Polyps erect miles high basalt towers vaguely comparable in internal structure to termite mounds. Once these creatures take root on a planet it becomes next to impossible to root them out. Like many other trains of Lovecraftian monstrosities once these thoroughly take root on a planetary body they will completely change, sterilize and lay waste to the planetary surface, and will introduce new forms of life native to their own original dimension.