To unlock the Hell Realm Cloak, you must destroy five Memory Stones. The following are the locations of the five stones. Keep in mind that all enemies in a location should be destroyed; otherwise, the stone you seek may not appear:
1.) Hill of Brimstone 1 (I) 2.) Hill of Brimstone 1 (II) 3.) Condemnation Tower (Level 2, East) 4.) Condemnation Tower (Level 3, West) 5.) Hill of Brimstone 2
After all five stones are destroyed, a sixth stone will appear. Head to the Inquisition Bridge to find and destroy it. The Hell Realm Cloak will then be yours.