Asphyxia resembles a mass of female bodies gruesomely fused together in a chain. She has a single pair of legs, which have hands for feet, and uses dozens of arms and hands to walk with, similar to a centipede. She also uses her hands to strike her victims. She lacks eyes and her mouth is usually covered by the upper hands growing out of her shoulders.
Nora Holloway was the daughter of Margaret Holloway and sister of Elle Holloway. Her, her mother and sister were descendents of the founding family of Shepherd's Glen. She was strangled to death at age 15 as part of the town's pact with the Order, a secretive cult responsible for many deaths in both Shepherd's Glen and the nearby Silent Hill, as a sacrifice to God (not to be confused with the Judeo-Christian God).
During Alex Shepherd's return to Shepherd's Glen, he finds Margaret Holloway strapped to a chair. He frees her and shows her a locket belonging to Elle. This locket, which originally belonged to Nora, triggers something in her memories that, coupled with the unknown powers behind both Silent Hill and Shepherd's Glen, summons her memories of Nora in the form of Asphyxia. He escorts her to safety when Asphyxia emerges from a fleshy hole in the wall, attacking Deputy Wheeler and pulling him in. Holloway then seals the door behind her, smiling evilly as Alex is trapped in the chamber with her daughter's monstrous incarnation. After a grueling battle, he killed Asphyxia by forcing her hands off her mouth, thereby forcing her to breath.
Asphyxia's name and appearance are symbolic of Nora's death. Nora was asphyxiated, or choked to death, in her case, by strangulation. The many hands covering her body symbolize how she was murdered. Nora loved to read, particularly Alice's Adventures in Wonderland, her favorite book. Having multiple arms, Asphyxia's form appears similar to that of the Caterpillar in the book and her "scream" is similar to how the Caterpillar blows smoke from his hookah at Alice. It is quite likely that this was another reason behind her form.