Opening Thoughts:
Dungeons and Dragons has long placed a great deal of emphasis on the fact that the souls of the dead will be sorted into specific planes of existence upon their death - the bulk of the Great Wheel is dedicated to this myriad of afterlives and godly realms, after all! But for all this, it never had a place to cover the actual sorting of those souls, which is odd considering how many other things the Wheel had.
The Land of the Dead:
The first half a page of this chapter is devoted to discussing what the Shadowfell is. It describes it as the world's twisted reflection, fashioned from excess "dark" creation-stuff that was cast aside by the Primordials, a secondary unintended echo, just like the Feywild. But where the Feywild is bright and colorful, the Shadowfell is dark and sinister; a realm of constant twilight, where darkness, gloom and melchanloy rule. This is not an evil plane, but it is a land of death and decay, with echoes of cities found in sprawling cemetary complexes or crumbling, oft-haunted ruins, whilst the terrain seems at once familiar and more insubstantial, darker, or ineffably frightening.
We're told outright that the Shadowfell encompasses elements of the Plane of Shadow and the NEgative Energy Plane from the Great Wheel, but it is more than the sum of its parts. A sidebar titled "Behind the Scenes: Hiding in the Shadows" iterates this point, but mostly it assures us that Shadow will become a future power source for 4th edition D&D - oe that, quote, "aids stealth, conjures illusions, inspires dread, devastates enemies, and manipulates necrotic energy".
The Shadowfell's ruler, or the closest thing it has to one, is an enigmatic goddess of death who inhabits a realm of eternal winter, for which she abandoned the Astral Sea and her fellow gods. Known only as the Raven Queen, she is the shepherd of departing souls and custodian of the dying. Her power is not unchecked, for she must contend with the machinations of Vecna, Orcus and Zehir, who all lay claim to this plane as well and seek to take it from her. Strangely, she rarely moves against them openly, and instead seems to prefer to subtly direct mortal adventurers to foil them. Why this is, nobody knows.
Not a Perfect Reflection:
This article talks about the major difference between the Shadowfell and its direct precursor, the Plane of Shadow - specifically, the difference in aesthetics. The major difference, as summed up by the subchapter, is that it's a gloomy plane, not a dark - you're dealing with dim light and muted colors, not an endless realm of blacks and grays that's virtually impossible to see inside of. This was also why the decision was made to emphasize the Shadowfell's environment as a twisted and macabre vision of the real world; to quote the paragraph:
A mountain in the world exists in the Shadowfell, but it might bear little resemblance: It might be taller, different in shape, or made of razor-sharp rock or some more bizarre material. If there’s a castle on the mountain in the world, there might be a corresponding structure on the Shadowfell mountain, or perhaps a ruin, a mausoleum, a mine, an encampment of monsters, or a gateway to a fortress in some other part of the plane.
The subchapter also discusses how this kind of geographic and visual fluidity can be used to recast your map - you know, if you're the DM - and essentially grants you two campaign worlds for the price of one. Well, three, considering that this can also be applied to the Feywild, but they're not talking about that in this chapter.
Death and the Afterlife:
As its title says, this subchapter breaks down how the "mechanics" of death work in the World Axis cosmology.
When an entity (at least a mortal one) dies, their soul enters the Shadowfell. Most souls depart very quickly - those called to the afterlife of a god's dominion are described as shooting through the Shadowfell like blazing stars in the night sky, as it's merely a veil that pass through between their origin and their destination. However, most souls do not go on to a divinely mandated afterlife - they don't go on to become petitioners, in the Great Wheel's lingo - and this is one of the great mysteries of the setting. Only the Raven Queen can clearly be said to potentially know where they go, or at least some details, but she keeps very tightlipped about it, and many whisper that even she doesn't know the full details.
Resurrection can only take place whilst a soul is still in the Shadowfell, or possibly if it is called to the realm of a patron. Souls that pass on are beyond the reach of any means to return them to life. Souls can extend the duration of their stay in the Shadowfell by acts of will, but this is dangerous; at best, it can cause them to transform into a ghost or a revenant; at worst, they can be corrupted into a monster or consumed by some foul predator.
Two final things of note that this subchapter closes on are this: firstly, divine afterlives are not inherently paradise nor damnation; they are generally appealing to a person who worships that particular deity, but they are not supposed to be a reward or a punishment in and of themselves. Secondly, most clerics and priests don't actually know what happens in the afterlife, but they simply preach their god's message and try to assure mortals that it is something to look forward to.
Why a Place for the Dead?
So, why the decision to add the Shadowfell? It has some rather obvious mythological precedents - I can immediately name the Greco-Roman Pluton and the Chinese Diyu, despite being pretty uninformed when it comes to mythology - but the Great Wheel never bothered with it, so why the sudden change?
Well, as this subchapter explains, the Shadowfell was added not just for its mythical resonance, but because it better meshed with the team's desires for the stories that D&D could tell revolving around souls and the afterlife. The Great Wheel did a lot of things, but when it came to classic storylines revolvring around the search for immortality, soul bargaining, and the quest for a lost soul? It didn't do those very well, if it all.
The distinct separation between "deity afterlife" and "land of the dead" allowed the design team to keep the gods as remote beings unconcerned with the mortal world, for the most part, without negating the ability of DMs to construct adventures around finding souls, resurrecting the dead, or constacting ancient souls for their knowledge. The mystery of what happens to souls after they "pass on" makes it easier to include reincarnation and the stories it can tell in the setting. Finally, it opens up greater possibilities relating to soul snatching and soul bargains.
Undead:
This is the first of five subchapters that begin thoroughly analyzing the undead creature type and its place in the new World Axis cosmology.
This topic breaks down the new necrology of the undead, establish that living beings are comprised of three parts: body, animus (instincts and the raw "spark of life"), and, for sapient creatures, soul. This phenomena will be expanded upon in the undead bestiary "Open Grave: Secrets of the Undead", but the basics are laid here.
Undead arise when mystical energy is suffused into an animus, allowing it to animate its dead body or even tear itself free and pursue a horrible new life of its own. Classic "mindless" undead have only the barest dregs of this charge; enough to grant them mobility, but no real thought. Most undead are comprised on just a "rekindled" animus, with or without a body; this is why they tend to be so monstrous in nature, because the animus is the seat of base instincts and lower drives, but lacking in "higher" spiritual concepts such as morality. The more power is invested in the animus, the more of a "mind" the creature has, but the lack of a soul leaves it twisted and warped. Wights are called out as the highest form of animus-only undead, but ghouls are another example. Shadows and wraiths are explicitly called out as disembodied anima granted undeath without their physical remains.
The "spark" that kindles an animus back into life can stem from many sources. Strong desires, beliefs or emotions are called out as potentially reanimating the dead, but the primary cause is necrotic energy from the Shadowfell. This shadowstuff is most obviously infused into remains by means of necromancy, but it can happen through environmental taint, usually in places that have been marked by a corrupting supernatural influence or which have become a "bleed point" for the Shadowfell.
Undead with Souls:
Despite everything said above, undead can exist that have souls, and they frequently do. Magical rituals, potent destinies and mighty wills can all cause this effect to happen, and the most powerful undead, such as ghosts, death knights, mummies, vampires and liches, are all possessed of their soiuls. This grants them much higher intelligence and greater access to whatever powers they had in life, which corresponds to their greater potency in undeath.
This subchapter also names the three entities most associated with undeath; Vecna and Orcus are obvious, but the Raven Queen is also tied to it, mostly through her willingness to permit the occasional soul to remain as a ghost or a revenant.
Despite the tales of undead monsters, and the evil natures of Vecna and Orcus, not all undead - certainly not those who retain their souls! - are actually evil.
Burying Dead Ideas:
This is the first of these grouped subtopics that focuses on a meta discussion. Specifically, it talks about the former association of undeath with the Negative Energy Plane, and how this led to problems.
In older editions, undead were described as being evil because they were infused with negative energy, which is the elemental embodiment of entropy - they were by their very nature antithetical to life, compelled to destroy and ruin. But then, as the designers note, how do you explain the existence of non-evil undead, such as the ghost? Why is the Negative Energy Plane the only elemental plane with an inherent moral alignment? Whilst assorted kludges were made, with 3e inventing the Positive Energy Plane-based Deathless, the 4e design team felt that this whole problem didn't need to exist.
This, then, is why they replaced the Negative Energy Plane with the Shadowfell, and created the new metaphysical necrology of soul, animus and body, which converted undead to a "keyword" rather than a "type".
Undead in the World Axis exist because they have been imbued with the energy to cling to the mortal world after death. Most undead are evil because they lack souls, or they attained their state by connections to (worship of) malign beings like Orcus and Vecna. But undead are not inherently evil, and the Shadowfell is no more inherently opposed to life than the Feywild is inherently opposed to death. The end result, so the designers felt, was that undead became much more malleable and twistable to DM decisions and designs.
Undead Everywhere:
This subtopic is the briefest of this quintet, and discusses one advantage of changing undead to a "monster keyword"; because it's now something that stacks over monster type, it's easier for the designers and DMs to create new and interesting forms of undead - undead angels, undead fiends, undead gods and elementals; all of these are workable in the World Axis in a way that they weren't in the Great Wheel.
Soul and Animus:
This sidebar talks briefly about where the concept of the animus came from, directly likening it to the Egyptian notion of the Ka and to other vague mythological sources, before reasserting and clarifying what I've been talking about for the last couple of paragraphs: the animus is the "base" part of the soul, it's the seat of animalistic desires and survival instincts. Granted new life and unshackled of the soul's conscious thoughts and higher emotions, it can be incredibly dangerous - anima-driven undead have no inherent morals to them, and even the smartest of such creatures lacks the inherent restraint of a normal person.
That is why undead are generally "evil". It's not so much that being undead is inherently evil, it's that most undead are created in a way that strips them of their higher reasoning and morality.
Shadar-Kai:
This final topic, I actually covered in depth in my Shadar-kai racial review, so I hope you'll forgive me for being brief. This article simultaneously sums up the new lore for Shadar-kai as descendants of humans who sought immortality in the Shadowfell and got it, in an unexpected way, and also explains the meta-reasons why shadar-kai were given a promotion as the major humanoid race of the Shadowfell.
Closing Thoughts:
Much like the Feywild, I feel the Shadowfell is a huge improvement to the cosmology. But unlike the Feywild, the Shadowfell is less filling a previously missing niche (although it is doing that, to an extent) and more taking two incredibly flawed planes and replacing them with something that, in my opinion, is a dramatic improvement. The Plane of Shadow never really interested me; I was there all through 3e and I never really grokked what it was about beyond just being an eternal night copy of the prime material. As for the Negative Energy Plane...
Spoilers warning: I basically loathe the Great Wheel's elemental planes. Most of them are, in my perception, boring, useless and absurdly lethal. They range from the endless ocean of mud to infinite empty void of space to the Negative Energy Plane, which manages to be even worse than the Plane of Vacuum. Despite the Doomguard and the Dustmen's ties to that Negative Energy Plane, I always found it to be the least interesting, least useful of the elemental planes. Even more so than the portion of the Plane of Magma or wherever it is that's described as an infinite desert of radioactive dust that gives you fatal radiation poisoning for which there is no cure and no prevention.
The Shadowfell takes the few useful ideas from both of its predecessors, boots them off the table into the cuttings bin, melds them with classic mythical underworld/land of the dead archetype elements, and makes something that works! You can see the resemblance of the Shadowfell to tales like those of Orpheus and Odysseus, but it's not directly beholden to them; it's tapping into a mythical archetype, and doing so very strongly at that, but still selling itself as a unique plane.