Soul mirrors are mirrors that reflects a person's conscience and actions and is effective in showing the true side of a person's soul. Everything a person does can affect what is shown in the mirror. Also known as synopados, it is generally kept in secret and has special properties to show the soul. With humans that belief is generally attatched to all mirrors.
The Gaunts have one in their house. However, other beings such as Elsewheres are invisible to the mirror. In the end of the third novel, Dybbuk Sachertorte examined his soul in the soul mirror, and saw it begin to deteriorate.