Seethers are a species of Empty - the memories of the souls that lie inside the Promyvionmothercrystals and can be considered fairly powerful with stronger weapon skills than a normal enemy. Like all Empty, they can associate themselves with any element, as indicated by its core glow, and it's weaknesses and resistances will depend on that element. Seethers can be found all Promyvion areas, and, like other Empty, can drop a Memory and certain Recollections.
Seethers defy ready description. They appear bipedal and almost humanoid from some angles, but the resemblance is slight, and if they are of humanoid ancestry, their time as memories has almost wiped such association away completely.