13th Floor Ghasts

The sound starts in the walls. A dry scraping, like fingernails drawn across the inside of a pipe, faint enough to dismiss as settling metal or vermin in the ducts. It grows closer. Then the grate shifts, and something thin and pale unfolds itself from an opening no wider than a dinner plate, and the only question left is whether you can get clear before those elongated fingers find your throat.
  13th Floor Ghasts are pipe-crawling undead, emaciated to the point of skeletal visibility, with claw-like fingers stretched well beyond human proportions. Their triple-jointed anatomy allows them to compress their frames through openings as narrow as one foot in diameter, turning every drain, sewage conduit, and water pipe in the urban ruins into a potential attack vector. No barricade, no locked door, no fortified position is truly secure against something that enters through the plumbing.
  Their hunting method is as simple as it is horrifying. A Ghast navigates the interior of pipe networks with disturbing speed, climbing through drainage systems until it locates prey above or beside a sufficiently wide opening. It emerges, latches on, and drags the victim toward the pipe mouth. The goal is not to kill outright but to pin the target against the opening, exposing whatever unprotected flesh can be reached.
  What makes Ghasts a persistent urban nightmare is the impossibility of fully defending against them. A settlement can wall itself off from the streets, bar every window, post guards at every entrance, and still lose someone to a Ghast that traveled through the sewage system from three blocks away. Blunt weapons are the recommended response, as their triple-jointed anatomy makes their frames vulnerable to crushing impacts.