They were once humans who were forcibly subjected to Viktor's demented experiments to extract their O Zhuvindo (life force). Viktor would later transform their bodies through the use of extensive surgery and the dark arts of alchemy, turning them into mindless meat-puppets that now roam his mansion grounds. Though the creatures still maintain some of their human senses, such as memories, feelings, and any sorts of humanity shortly after transformation, they have no free will to control their own bodies and their mind would eventually deteriorate in time, leaving only a desire to feed on the living.
These creatures are shown to be different from traditional undead in other ways - while Zombies are usually stopped dead with a shot to the head or the removal of it most if not all of the creatures in Rise of Nightmares can survive, and continue actively to pursue Josh, after having their heads decapitated. The only to kill them is to completely dismember their bodies until they drop dead. The creatures displayed little intelligence, though they are capable of complying to their master's bidding.
The creatures are rotting, undead corpses bearing drastically visible scars as a result of Viktor's inhuman tortures. Most of the creatures seem to wear ragged clothing that presumably belonged to them when they were humans, however some of them even wore many unique outfits such as, a nurse's and an orderly's uniform, a maid's dress, an ancient cultist overall, a formal dress, a guard's uniform, a chef's outfit, and even a gimp suit.
All of them are all fitted with odd-mechanical body parts which they use primarily as weapons, however some of them also have different attack methods, such as spitting acidic vomit to obscure the player's vision, blowing themselves up through the use of the strange vat implanted in their abdomen, shocking the player using the electricity surging through their body, or create powerful rantings.