Unlike its flourishing cousin, Cambrius, the Grotto System is still reeling from its experience during its Compliance. Before the arrival of the Imperium, Grotto was one of the top tier system Kingdoms which formed a vast network throughout the Sub-Sector; it was also one of the Kingdoms which refused to bend its knee to the Emperor. The Imperiums response was swift, devastating the inner the worlds of the System which showed the most resistance.
It was during the Compliance of the innermost three worlds that it was discovered that the human Kingdom was actually a puppet regime controled by a shadowy subterranian Xenos race situated on those worlds. The systems 'capital', Grotto, was deemed clean of the Xenos race and so only one in two of its population was executed in Penance, while the triplet Styges planets recieved the full brunt of the Astartes assaults. Although officially declared compliant and the Expeditionary fleets long ago departed, all three inner worlds are still at various states of conflict with constant battles raging in the underground catacombs as the last of the Xenos are hunted down by Sub-Sector forces.