The Fearful are an alien race first encountered on the "living planet." Since that time, they have changed, merging with the "grey goo" nanitic substance introduced when Max Cohen visited, prompting a fundamental shift in the aliens’ culture toward agressive annihilation of the humans and other mystic-magic-wielding races.
At one level, the Fearful consist of a non-sentient virus that infects whatever organic life it touches. However, once an entire ecosystem has been infected it acquires sentience, with each ecosystem corresponding to one individual. To communicate, individuals swap pieces of their ecosystems with each other.
Instinctively, the Fearful "know" that mystic energy is evil. It must be purged, or barring that, escaped. A universal religion was created around doing so. Since their merger with the grey goo, the Fearful have shifted substantially away from "escape" and toward "purge."