The Haunted Mask was created by a nameless shopkeeper long ago. The shopkeeper had no love for himself, for he was born with an ugly face. As a grown man, the shopkeeper became a master in Alchemy, and in turn used this to create a beautiful human prosthetic face. He made this face from the flesh of monsters and made it so that when he put it on, it would attach itself to his flesh and would remain so forever. But something went wrong. When he put on the beautiful mask it changed, it became hideous. No matter what he did to stop the decay, the face turned into an ugly monstrosity. The face turned a dead green, its teeth grew into animal fangs, and its flesh became contorted and deformed. The shopkeeper did the only thing he could do. Knowing that the face he made was never supposed to be removed, he took a knife and carved deep into the skin where the mask was supposed to end. This first failed attempt became the Haunted Mask.
After painfully removing the mask, he tried countless other times to create the perfect flesh mask for him to wear to hide his face. They all failed just the same. It was after his failures that he realized that the reason the masks had failed to remain as they were created was because they changed his outside appearance but not the way he still felt inside: ugly. These feelings in turn were what transformed the faces. They changed to reflect the man he still was on the inside, hideous. The shopkeeper took his failed attempts and put them on a shelf in the back room of a mask shop that he was still working on setting up. He put them there as a reminder of what he was still trying to accomplish. Meanwhile, in the back room of the shop, the first mask, the first failed attempt on the shelf waited and watched, for the day it would gain a body.
However, that day didn't come, as R.L.Stine, Elsa, and a young Stranger, came to the town, and to the store. Hearing rumors of the evil of the mask, they had asked for the storekeeper's help to seal it forever. The fate of the other masks was that they were burned, but that the First Mask was too powerful to be full destroyed. And so it remained in the book.Until when Courtney set it and the other creatures free.
The Mask itself is a parasite in ways, able to latch on to people, and encase their bodies in a full-body suit of its own skin, slowly transforming their outer appearances. It can stretch itself, making tentacles of its stretchy flesh, as well as grow spikes as defensive and offensive weapons. It can also change the host/victim's personality, making it bend to its will. It also appears to have the ability to birth or spawn other parasitic masks, servants to its power. Even for a short time, the Mask can stretch itself, forming a temporary body.
Its weakness is Love, a symbol of love. Love was the one power that could fight off the darkness that the mask gave off. As that love grows, the Mask's power grows weaker.