Veil of the Abyss

Publisher's blurb:

Nakar-T’har, a bleak, cold prison planet floating in the void, was established by the last remnants of the surviving Civs. The prison holds the most dangerous criminals, dissidents, and exiles, but now, an ancient force – something unseen and unexpected – has begun to awaken beneath the surface. The planet’s iron fortress was meant to be unbreakable, but something older than time is eroding its walls from within. Those who live in Nakar-T’har have learned to survive brutal conditions, but not even the toughest among them were prepared for this.