7. Gaming
How might you build this setting into a game? Obviously there's a special invitation to @dr_mitch to elaborate on his thoughts about this, but others might have different views to complement his.
Which elements from the book would you include and which would you drop? Would you focus more on magical combat? Investigations into mystery? Crime with magical elements? Negotiations between supernatural beings?

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This is not the kind of setting I've ever wanted to game, so I have to admit I've never really thought about it before. I'd tighten the focus - police and spell-casting would be more than enough. Police and ghosts is enough. Maybe I'd do spell-casting and ghosts. But Deptford D&D? No thanks.
I'd also be inclined to make it edgier, with more of a horror element. I did quite like the idea of 'your brain on magic' and faces falling off, so that would be in. I suppose that, like in the Call of Cthulhu RPG, I'd make a cost for using magic. All in all, it would come off a lot more like Ron Edwards' game Sorcerer than Deptford D&D.
The "supernatural police procedural" genre is one that could be interesting, but I'm not sure how well police procedurals translate into RPG terms. There's a lot of knowledge required about what police procedures are in order for the games to work, and there's that whole issue of how to do investigations in an RPG without it bogging down into a puzzle for the players, not the characters.
I think that if I were to do a game like this, I'd probably base it on the ideas behind things like Unknown Armies or Don't Rest Your Head and make the game revolve around the obsessions of the PCs, rather than investigations into mysteries.
And @Apocryphal , take a look at the London Falling recommendation I made in another thread in this category. I think you'll like it.
What appeals to me as a modern day supernatural setting is the supernatural elements having a strong sense of place, and drawing on that place's history and folklore. That's what I did in Liminal, with doses of invention, and that's what this book does too to a great extent.
The magic in Liminal is much less flashy. The magic in the Rivers of London series reminds me of Ars Magica as much as anything.
Liminal isn't a police procedural though - it's more the private investigator equivalent when looking into the supernatural, with the PCs being potentially partly supernatural themselves; they are caught between the mortal world, which is largely ignorant of the supernatural, and the Hidden World where such things are of primary importance. Though there is a police division involved into looking into supernatural crimes.
Sorry for banging on about Liminal here, but as the question amounts to "how would I game it?" I've already written my answer!
Please don't be sorry... I for one wanted to hear a bit more about Liminal and the choices you had made.
With "the supernatural elements having a strong sense of place", are you saying that, for example, the various river deities would find a parallel in Liminal, in the sense of tutelary spirits linked to a particular location?
Yes, there are definitely parallels with river spirits, though in Liminal they are particular instances of lords of the Fae. Generally in Britain there's a north-south divide, with the most powerful Fae being the Queen of Hyde Park and the Winter King. Mother Severn is also powerful, as is the Eveling King of the Lake District.
Sites of historic trauma tend to spin off ghosts and ghost realms, which are almost pocket dimensions which reflect the time past when the ghosts were around, and are tricky to escape.
Having not heard of Eveling before now, I did some searching and rapidly found
https://esmeraldamac.wordpress.com/2012/01/11/eveling-cumbrias-faery-king-and-celtic-god/
which others might enjoy.
I particularly liked the connection between Ravenglass and Hardknott Pass - it is a truly spectacular drive up from one to the other on a fine day - and as @Apocryphal will remember, Jack Whyte also connected the two in his version of the Arthur/Merlin story.
In fact I recently came across a photo I took many years ago. The walls of the Roman fortlet can be seen at the crest of the foreground hill.
A fine pic, on a fine day! I've often thought when passing there that if I'd come up from Rome I would like the Hardknott Pass on a day like this, but not so much when the icy winds are whistling down the pass, and the frozen fog is seeping into your toga...
Like dr_mitch my urban fantasy gaming would be tied up in my game.