He's a manifestation  -- I've grasped that much. A big, darkened manifestation of this black substance (which my friends and I affectionately refer to as ink). But it's not really ink; it's a thick, malleable substance that can bend and shape into any form in pleases, commonly taking on the form of tentacles to snare and trap it's prey. 

 Prey.; Disease preys on children, and it utilizes many creative ways to lure them. Sometimes it will wear a pure white mask that in a low light can form the facial features of the child's mother, it uses this illusory trick to fool the child into coming closer and it can move both in a quad and bipedal state, which leads the child to believe it's lie even more. (The 'mask' design was drawn from a foreign folklore I read about a creature that would wear a white mask [with or without features] to trick children into believing it was safe. White is the color of purity, and to most children a friendly, hospitable face would lead them to follow them.) The creature's arms and legs also resemble human hands at the ends, so it can pick up small of objects of comfort or stuffed animals or even use them to scale walls and ceilings -- which it will resort to if the child tries to run -- the 'tentacles' also come in handy in this situation. Once the child is close enough the 'mask' will seem to gradually fade, then when they are at an arm's length they are faced a a giant, oozing, coursing, ball of thick, black, ink; and then the creepy part, a mouth manifests out of the creature's chest to gobble the child up whole. 

 Now, being a manifestation gives the creature free will to produce eyes, limbs, teeth, and it's ghastly 'mouth' as it pleases (sometimes the mouth will manifest where it's face should be, or on it's side). The fact that it lacks a stable body also enables it to switch between solid states and 'break down' into a puddle of black goo that can slither under cracks in doors, into the air vents, and even through the small cracks in a screen window for those parents that think they should leave the windows open at night. The most important thing about the creature is that it will never make it's self visible until the child realizes it's alive. This can be done by hiding out on the ceiling and dripping ink onto the floor, or leaving trails on the walls that would lead a curious child to investigate. Houses are picked at random and it mainly just slithers into the closest one when it requires a little snack. 

he takes on the form/stature of an omni, a big, inky omni with tentacles. think no face meets slenderman