Sentinels are the largest creatures in the Ultravoid, made out of bone and sinewy sludge. They are enemies that the player will eventually have to battle in order to progress in Rei's quest to save her homeworld from the Ultravoid.
Anatomy
Sentinels exist in many shapes and forms, though all of the known ones appear to have long bodies. They are massive remnants hewn from white bone and black and pink viscose, tendriled, sinewy sludge. Their bodies are mostly made out of the sludge, with the white bone being like armor plating on their bodies. It appears the bone-less part of their body is the most vulnerable.