While Hallow Ones can definitely be considered “monstrous”, they are not technically monsters themselves.
Hollow Ones start as normal humanoids that belong to the evil cult of Shura. After years of mental conditioning, the humanoid in question undergoes an incredibly painful ritual which involves them being strapped down to a rock slab. After a short chant, the victim has a hole drilled into their chest in which the larvae of a carnivorous Dusk Worm is inserted. This begins the extremely painful, physical bonding process.
Over time, the Dusk Worm eats away at the insides of its host and takes control of their body and mind, which effectively makes them into a hallowed out zombie, as the host’s skin and flesh start to rot over time. As time passes, the worm grows larger and larger. The Hallow Ones feel no pain and are incredibly difficult to kill. The only known ways to destroy them are by severing the head of the host or by destroying every last trace of the Dusk Worm inside the host (which is difficult since they have regenerative powers). In addition, the Dusk Worm is able to come out from it’s hollowed out host at will, and do as it pleases.