Though Simmons first thinks the planet's habitual sandstorms are nothing more than a normal environmental occurrence, she soon learns that they're a calling card of the planet's reigning evil, a cloaked figure who kills or drives mad everyone it encounters. Simmons hypothesizes that this creature has been receiving human sacrifices through the portal, much like those in which the Gloucester men in 1839 seemed to be partaking.
As to how Simmons avoided falling prey to this being? It turns out that she had some help, which is exactly why she plans to return to the planet with Fitz's (Iain De Caestecker) assistance. But Fitz isn't exactly thrilled about this plan for one major reason that has nothing to do with the danger involved.