An unknown disease strikes the Underground. (Needs first victim). Alphys studies the disease, and the process starts as the monsters falling in a deep comatose-like state where the disease has time to feed and grow. With the weakened state of body, the disease can grow more rapidly. What grows inside is more like an infection or tumor, which leaks black liquid, causing black liquid to drip from (and is not limited to) eyes, nose, mouth, ears, and any other cracks or holes. The tumor is more like a parasite and 'eats' the magic within the monsters, slowly killing them, but at the same time acting like a life force so they are in a near-dead state, or undead. Skin/bones become discolored, fur falls off, parts get weaker and more susceptible to breakage. Since monsters don't decay and are made of magic, they take on a melted amalgamate-like appearance. Once the disease fully takes over, it has a mind of its own and the monsters are controlled by the parasite. However, they can still think, but are too weak to move themselves. They can, however, still speak a few words - for example, 'help me'...
(...showing somebody my Undertale AU o.o)