"That makes sense. And conquering Fairyland is, if not easy, at least straightforward."
In his ongoing search for information on the Gem of Amara, Sherlock finds a few books he thinks Promise might like and adds them to the stack with the scrying books. On top of learning a lot of things about various kinds of scrying, she can also read about various kinds of portals or other methods of moving between worlds.
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.
"It seems like gates are preferable for most purposes, especially if I can get them to go to arbitrary demon dimensions should I wish to travel there."
"For most purposes," Sherlock agrees. "But if we get our hands on some means of travelling between dimensions that doesn't have the settling-time problem but can get us to Fairyland, that could turn out to be very useful."
"That's what I was thinking, yes. That and raiding assorted hell dimensions for useful recruits. A lot depends on which spells and artifacts turn out to be practically accessible to us, but interdimensional transportation could be a promising avenue on multiple levels."
"I'd rather not make my stand against slave labor with the use of slave labor."
"Well, that's fair, but it seems at least possible that you might be able to find more people who are interested in helping of their own volition."
"Possible. I have... no personnel management skills that do not derive from watching Thorn."
"And I'm not much better, but there's got to be some better solution here than 'oh, well, slave labour or nothing'," he says. "Further thought may be required."
"Are you anticipating getting some specific sort of use out of hell dimension residents as opposed to standard humans?"
"Access to a wider variety of magical traditions. Assorted useful magical properties. Higher chance of stumbling across someone awful enough that you're willing to enslave them and powerful enough to be definitely worth enslaving."
"Different cultures develop different ways of doing magic, usually with different specializations and underexplored areas, often appearing to work on completely different principles, yet bizarrely cross-compatible whenever anyone bothers to learn more than one. There is, as usual, no credible explanation for this whatsoever."
"I think I like sorcery better, but it seems like the magic here can do more different things and from more flexible initial conditions."
"I'm not familiar with sorcery and I'm only very broadly familiar with the local stuff, but that fits with the impression I've been getting."
"I might offer to teach you if there weren't so much else to do and if it didn't take years to get anywhere very useful and if you weren't devoid of moral feeling and motivated entirely by boredom."