The Interstice is a blended space between the physical realm and the Astral Plane from which creatures spawned from the subconscious will of humans and deceased spirits reside. Compared to normal humans who would have a firm hold on their plane of existence, certain humans that use magic (like witches and wizards) or those branded with a Brand of Sacrifice (like Guts and Casca) have a footing in the Interstice and can interact with the beings residing there. The Skull Knight also possesses the ability to exist between the two realms.