Shadekith

Creatures of animate shadow and negative energy, shadekith are born of darkness and the absence of life. Their mentality is alien and unknowable, they speak with each other in a language indecipherable by any not native to the plane of negative energy or the Shadowfel, and they seem driven to torture, destroy, and consume all creatures that don't share their own physiology. Even undead, which classically are viewed as a kind of pseudo-relative by creatures from the plane of negative energy, are not immune, as shadekith are all-too-happy to dismantle them and devour the negative energy animating them.
The only "good" thing about these creatures is that they are incapable of reproducing outside of their home plane, and so the only way for them to truly cause wide-scale destruction would be for a long-lasting portal to the negative energy plane to be established - an event which has a multitude of other terrible implications, with a shadekith infestation representing a meager portion of the attendant devastation.
Shadekith resemble the ethereal undead that are called "shadows," and share some of those spirits' energy-draining powers. However, shadekith are physical creatures, with flesh that feels like "a freezing-cold sponge soaked in oil," according to one survivor. Their physical shapes vary from humanoid to insectoid, and everything in-between, with very little difference in their capabilities between shapes. Due to this, they are normally classed by size, with the smallest versions being termed "shademites," the mid-sized ones "shadekin," and the largest, which can reach the size of a hill giant, "shadowgren."