A liche is a paranormal being, a product of the Magick Realm.
In the files of the Ministry of Occultism is a document labeled Liches and Wraiths describing supernatural creatures:
A Liche (also Lich, or Lych) is usually a human or equivalent who cheats death by placing their soul into an object separate from their bodies. Henceforth the Body can only be damaged by damaging the object that houses the Soul, while direct assault has no effect. That said, it seems the body can be physically altered to a certain degree - it can be stretched and twisted, but not broken or torn. The Liche's body does undergo decomposition but after reaching a point of near-skeletal decay this process seems to halt. Liches are extremely reliant on magick, as it is the only thing holding their rotten bones together. Cabadath is the only Liche thought to be active in the Scientific Realm at present.