I had a look at some information for Tome of Magic: Pact, Shadow, and True Name Magic. It looks like a third of the 285 page book is devoted to Shadowcasters (and new spells, feats, prestige classes, monsters, and magic items that tie into that theme).
Here is a bit from the product blurb that shows how they work:
What she was proposing was to make similar types of spellcasting that work with other planes, and I spoke about that over there, but I think it might even be possible to make an entire bespoke crystal sphere that just uses Plane of Shadow themes (including Shadowcasters).Tome of Magic: Pact, Shadow, and True Name Magic blurb wrote:Shadow Magic
The Plane of Shadow is a dark, twisted reflection of the real world. The shadowcaster, by understanding the fundamental properties of the plane and unlocking it's magical mysteries, learns to harness and channel its umbral gloom, shaping the darkness to server her whim.
Richard Baker, who put some Spelljammer content into his Blades of the Moonsea novel, Corsair, but he also had a couple of sections that show how the "dark, twisted reflection of the real world" thing can work. In his story the protagonist travelled to a Plane of Shadow version of his hometown, while chasing a foe. Anyhoo, I think that the idea of a crystal sphere, where some locations had Plane of Shadow duplicates could be a lot of fun. (Just imagine if a place like the Rock of Bral, had some sort of group operating from the Plane of Shadow. On that plane, they could be living in Prince Andru's palace and the Lower Market could be a slave market. And nobody would even know they were there!)
Forgotten Realms uses an alternate spellcasting source called the Shadow Weave. There is a WotC article called Class Chronicles: Shadowcasters that explains a bit about how the class might fit into the Forgotten Realms (and used alongside the Shadow Weave). But I'm not totally sure I'd want to steal things that are iconic to Forgotten Realms, when Shadowcasting by itself would have a newness to it.
The Shadowcaster is an interesting concept (that could probably also be retroconverted for 2nd Edition Spelljammer games).
I know that Shades come from the Plane of Shadow.
Can anyone remember any other shadow-themed things that could fit with this idea?