Ellen's Knife (エレンズナイフ Erenzunaifu) is an item critically needed for obtaining the True Ending in The Witch's House.
AppearanceEllen describes it as a small knife used for crafts.
Upon discovering that her mother abandoned her, Ellen grabbed this knife and stabbed her repeatedly in the throat, killing her. She later killed her father with the same knife as he was mourning his deceased wife. After the incident, Ellen kept on grasping onto the knife as if were a part of her, as if it were her "cat's fangs," and eventually met the Black Cat when she sprawled on the streets due to pain and fatigue. When she became a Witch, Ellen was still holding onto the knife. It can be therefore surmised that the object held some witch-like powers as well.
In her own body, Ellen is able to magically materialize the knife in her hands at will.
Ellen's Knife is found in the Present Room on the First Floor, where it is located in the back closet. The note on the closet initially says, "OPENS WHEN THE HOUSE RETURNS TO NORMAL." Make sure to retrieve it during the scene when the Legless Girl chases you if you want the True Ending.
Aside from taking up inventory space, Ellen's Knife has no real use as a key in the house. It can only be retrieved at the end of the game, while "Ellen" is chasing you.
Its only purpose is to unlock the True Ending. It will be used to stab the eye socket of Legless Girl while Viola has a conversation with her.
It should be noted that the old news clippings in the second floor library read that Ellen's parents died due to "stab wounds." Along with the Witch's Diary entry where she says that she X her parents, it is surmised (game-wise) that this is the knife Ellen used to murder her parents.
This unlocks the "True Ending" of the game, showing that throughout the game, Viola's body has been taken by Ellen, who was the real witch.