The Mortal Realm is the name given to the universe in which Engelain exists. The Mortal Realm consists of the planet upon which Engelain resides, the sun, moon and other planets, and the stars beyond. It is unknown where the Mortal Realm ends physically, but it is defined by its physical constraints. If you can travel to a place without use of some form of portal, then you are in the Mortal Realm.
The Mortal Realm has an entwined relationship with the Underworld, where all mortal souls go. The Halls of Janus are also intrinsically connected to the Mortal Realm, though the nature of this connection is both unique and baffling.