Complex sentence: Cut off from communication with the other worlds (Chelestra specifically and the Council), and slowly dying of some unknown cause, quickly becoming vastly outnumbered by the mensch, the Sartan there settled into hibernation, hoping for help from the other worlds.
Split into simpler sentences: Cut off from communication with the other worlds (Chelestra specifically and the Council), the Sartan were slowly dying of some unknown cause, quickly becoming vastly outnumbered by the mensch. The Sartan settled into hibernation, hoping for help from the other worlds.