Devour is a Deep Sea Demon from the species Eye of Branch Nest, which through the aftermath of a fight with Nightingale, got separated from the Mother of the Nest and now seeks to devour everything and evolve for its own benefit and survival.
At first glance, Nightingale described it as a gigantic red plate, very much alike to the "Bloody Moon", but with tens of thousands of red eyes that looked like stars. Compared to Eye Demons, it appears as if the demon's body had been flattened and considerably stretched. It didn't quite look like a creature, so much so that it didn't have epidermis or muscular tissue, and it seemed to be made of vascular intestines, tentacles and difficult to name organs all stacked up. It looked unstable and horrible.
It appears to have several brains which store pheromones inside it.  After several years worth of evolution, it separated its eyes from its organs and attatched them to its new grown skin and upgraded its ribs into a capace filled with magic power, which amplified its defensive capabilities. It also grew a neurotoxins' filled stinger, and grew three times in size.
Initially, it had no self-made desires, it only did what it was created to do, without any questions. However, after it fought Nightingale it got separated (mentally and magicaly) from the Mother of the Nest and it started to feel fear for its life along with anger against the one that made him feel physical and psychological pain. It also started to demonstrate egoism: since it wanted to live it avoided going back to the Zenith Sea to be anexed by the Mother of the Nest. Eventually, it starts to see itself as something different than a usual nest eye and started to value survival above everything else. Evolution was the only way it found to remain alive. Also, it doesn't seem to care that much about getting injured due to battle, as long as it can regenerate itself.  After years of evolution, it eventually started to have feelings of joy. 
Not much is known about it before his encounter with the witches  except that since he was an Eye of Branch Nest, then he probably traveled around the world collecting pheromones of the various beings (which he calls them equally as bugs) it attacked and engulfed to eventually bring them to the central nest. 
Its first appearence in the story occured when the First Army, the Witch Union and the Taquila Witches went to explore the underground of the Great Snow Mountain. It was first sighted by Nightingale , and sent various Demonic Hybrids after her when she appeared again but the wounds inflicted on her by them were not able to kill her. Due to that, half of its body got blown up by explosives the witch put in it. It fled to the river and eventually found himself in the ocean.
While injured, for the first time it felt fear and anger, and through that grew as an individual separated from the Mother of the Nest. Ten days later, through its regenerative power it repaired itself, but realized that it had lost many pheromones (indicates possible evoluiton directions, each from different types of being absorbed) from the attack against it. It eventually found out that part of its brain got blended with the red mist insect which gave him his regenerative powers. Through this blending, it developed its own sense of self and started to seek for evolution for itself.
Due to years of evolution, it changed its appearance dramatically and it eventually started hunting other Eye of Branch Nest to continue its evolution. Around this time it felt the need to distinguish itself from its new victims and gave itself the name "Devour". 
It is later found by Roland while probing the The Cradle anxiously cowering into the sand at the bottom of the ocean, occasionally sneaking a few looks around with its eyes. However, he is left alone, to eventually compete with the Human Civilization. 
Powers & Abilities
Due to its similar features to the Eye Demons, it can sense and locate their enemies upon being noticed, even if its enemy is invisible. It seems to be able to control Demonic Beasts and Demonic Hybrids through its roar. It had several slender tentacles, some like steel whips, others with diverse magic abilities.
It possess a very fast way to adapt to survive to new conditions, due to being able to anex all manner of beings to itself after consuming them. This provides it with possible evolution paths it can follow and quickly implementing it, through the modification of its body. It also absorbs magic power through this process of fighting and anexing 'bugs', which eventually leads to its evolution and sharpening of its senses. It possesses an extreme power for self-regeneration due to the absorption of a red mist multi-eyed insect (probably a Demon, most likely the Eye Demon).
After several years of evolution, it started producing a neurotoxin that when inserted with his stinger fatally wounds its target, while preserving its body to absorb its pheromones. It also now made magic free silk-like explosives to attack from a distance gallstones and bone spears. 
- It only appears to fear two things: dying and the Mother of the Nest.
- Chapter 811-813
- Chapter 807
- Chapter 813
- Chapter 845
- Chapter 1366
- Chapter 1494