A book about the Hellmouth itself and other location-specific, plausibly-naturally-occurring dimensional portals, discussing their respective characteristics, which realms they are known to connect to, and how flexibly they may be used to travel to places they do not naturally go.
A book about known demon dimensions, discussing how easy they are to reach, what it is like there, and the nature and hostility level of their inhabitants. This demon dimension is normally mutually accessible from Earth only at specific sites; this demon dimension is inhabited by a non-hostile species who can travel fairly casually between realms but who refuse to allow any visitors and don't often leave their own dimension in the first place.
A book on advanced magic that has a section on (difficult, dangerous) portal creation alongside sections on (difficult, dangerous) forging of magical weapons and (difficult, dangerous) ritual warding.
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A book about the Hellmouth itself and other location-specific, plausibly-naturally-occurring dimensional portals, discussing their respective characteristics, which realms they are known to connect to, and how flexibly they may be used to travel to places they do not naturally go.
A book about known demon dimensions, discussing how easy they are to reach, what it is like there, and the nature and hostility level of their inhabitants. This demon dimension is normally mutually accessible from Earth only at specific sites; this demon dimension is inhabited by a non-hostile species who can travel fairly casually between realms but who refuse to allow any visitors and don't often leave their own dimension in the first place.
A book on advanced magic that has a section on (difficult, dangerous) portal creation alongside sections on (difficult, dangerous) forging of magical weapons and (difficult, dangerous) ritual warding.