I've seen this come up multiple times in different sources, but I've never seen any proper explanation about what it's supposed to be. I know it's magic associated with the Shadowfell, but that's about it. Plus, it's effects seem all over the place: For Illusion Wizards it's used to create temporarily physical illusions from some kind of extraplanar matter, for Shadow Sorcerers it turns them into pseudo-undead and lets them conjure effects from the dark, and according to MTOF the shadar-kai use it to steal memories.

The way it manifests for Sorcerers makes the most sense for me... except not completely since I thought negative energy was the source of undead phenomena, unless shadow magic and negative energy are related?

How should I understand it?

For a quick reference, this may help: https://forgottenrealms.fandom.com/wiki/Shadow_magic

Quote from Seldarine801 >>

For a quick reference, this may help: https://forgottenrealms.fandom.com/wiki/Shadow_magic

 I dunno. Last I checked, the forgotten realms wiki hasn't really updated much in regards to 5th edition lore. 

Honestly there's very little theme to the things that WotC is pasting the name "shadow" to these days.  If it's kind of edgy and maybe a little creepy, they attach the term shadow to it can call it a day.

Quote from Lyxen >>

Agreed, they have even very much simplified the Shadowfell, and sort of merged it with the Plane of Shadow. It's not exactly clear if they left the positive and negative plane exist or not in the standard cosmology, some monstres (Nightwalker) hint at least at the existence of the Negative Plane, for example.

As I like my cosmology complex and rich, I usually keep all the planes separate, but I agree that it might not be completely RAW in terms of Shadow Magic. But I like using the Feywild and the Shadowfell from 4e where they were the most developped and the Plane of Shadow as a transitive plane as in 3e, the Manual of the Planes from that edition is really cool.

 except it is very clear that the Positive and Negative Energy planes still exist... you can see them in the planar map in the PHB, and even have a direct call out in a box.

"POSITIVE AND NEGATIVE PLANES

Like a dome above the other planes, the Positive Plane is the source of radiant energy and the raw life force that suffuses all living beings, from the puny to the sublime. Its dark reflection is the Negative Plane, the source of necrotic energy that destroys the living and animates the undead."

And here in this map directly from the PHB, Positive and Negative are on the outside of the wheel, clearly labeled. 

Quote from Lyxen >>

Thanks for this, I mist say that I had missed them despite an even clearer diagram just above in the section. I can only say that I was not expecting them to be there. The Positive and Negative planes for me have always been inner planes, I was not looking for them on the outside of the outer ones. :D

I must also say (but it might just be the traditionalist in me), that I don't like it, for me they are more energy planes and their interaction with elemental planes as part of the inner planes made sense, as making the Shadow Plane a transitional plane made sense as well, whereas (coming back to OP's point), it certainly cannot exist any more in this cosmology. And you would expect the positive plane to be linked to the Feywild, and the Negative one to the Shadowfell, otherwise it starts to be confusing where the source of undeath is.

 oh no, I fully agree that they should be Inner Planes, and I deeply miss the Quasi and Paraelemental planes that have not yet made an appearance in 5e. But oh well.

Quote from Darkmoon250 >>

Quote from Seldarine801 >>

For a quick reference, this may help: https://forgottenrealms.fandom.com/wiki/Shadow_magic

 I dunno. Last I checked, the forgotten realms wiki hasn't really updated much in regards to 5th edition lore. 

 True, in part because there hasn't been much update with 5e on this kind of thing. 

Shadow magic was a type of Weave-based arcane magic that channeled power from the Plane of Shadow. The Plane of Shadow can be a hard concept to get because d&d has changed their cosmology models between editions. Originally it was “The Great Wheel of Cosmology” then in 3e it changed to “The World Tree” model, then in 4e It changed to the “World Axis” model where it split half into the Astral Sea and the Elemental and Energy Planes collapsed into the Elemental Chaos forming the other half. But not before Shar In the forgotten realms, managed to manipulate some of the necrotic energies from the Negative Energy Plane and inject it into the Plane of Shadow, which created the “Shadowfell”. So the Shadowfell is a parallel plane to the material plane (like the Feywild) made from the Plane of Shadow and part of the Negative Energy Plane. During this time, until the Second Sundering, the parallel plane (Shadowfell) became the place from which necrotic energies and shadow magic stemmed. Now in 5e I think they have reset back to The Great Wheel model, which has Both a negative energy plane and the Shadowfell. In remote corners of the Shadowfell there were demiplanes created by the Dark Powers which served as prisons to trap creatures of extreme evil to serve as sustenance. Like Barovia where Strahd lives.

So Shadow magic was but one of many ways of accessing and controlling the natural energies of the ”Weave” and was strongly tied to it. Practitioners used energy taken directly from the Shadowfell (Plane of Shadow) and used that energy to manipulate “The Weave” so they could cast spells. But If you don’t play in forgotten realms this may not apply to your setting and best to ask your DM in that case.

You are correct Lysen. Yes it’s a Forgotten Realms View. That’s why I said If you don’t play in the forgotten realms this may not apply to you and ask your dm. Although if you are the DM you may not have anyone to ask. #foreverDMproblems

And Darkmoon250, Forgotten Realms doesn’t explain it the best if you are on a different world. I’m not sure any of the different spheres explain it as fully as we all would like. And it certainly hasn’t been translated well across the editions with the change of the play systems. (I don’t miss THAC0 that’s for sure).

I think I read somewhere that Shadow Magic was left deliberately vague and confusing in the forgotten realms due to Shar’s tricky & secretive nature. Can’t remember where I read that though. Practically it was probably left so open to allow DM’s to explain it however-they-so-desired in their own games. If you don’t play in the Forgotten Realms (heck even if you do) I think you are fine picking and choosing from the lore to make a continuity that works for your home game and world. 

If you do play in the FR’s (heck even if you don’t) then feel free to use some, none or all of what I said. A great source for info  if you are in the Forgotten Realms is Ed Greenwood himself. The guy is awesome on his Twitter answering Lore questions and he is a really sweet and kind man. 

Quote from Lyxen >> subject.

Personally, I will stick with my own preference, i.e. that shadow magic is linked to the shadow plane, that this plane originates from the interaction between the positive and negative plane (both inner planes), and that it makes the prime(s) cast duplicates, one more bright being the Feywild and one more dark being the shadowfell, where actual shadows congregate.

The shadow plane is a transitive plane, which is coterminous with both the prime and the feywild/shadowfell, allowing it to be used as a travelling medium (as an alternative to the ethereal plane), but it also allows stuff from shadow to be made manifest in all these planes. When it's low power, it's just mostly illusions, but if you are powerful enough, you can give substance to your shadows and create all sort of magical effects from the shadow "stuff". It's of course easier to do in the shadowfell (advantage?) and really hard to do in the Feywild (disadvantage), with the prime being intermediate in difficulty.

 Sounds Great. Good flavour, I like it.