The Warcraft Cosmos is aligned along three major dichotomies. Light and Shadow, Order and Disorder, and Life and Death. Most of these have longstanding, well-established beings of incredible power. Naaru, Titans, Demons, Old Gods, Void Lords (the Shadow gets two) and the Wild Gods (which, to reiterate, is the blanket term for Ancients, Loa, and Celestials, and any other animal gods we come across) each represent their Great Power.
But for Death, we simply have "The Undead."
The Undead in Warcraft arise in many different ways. Probably the most common are simply ghosts - lingering spirits of mortals who died and are somehow still bound to the general location of their demise. In some cases, these are specifically caused by traumatic magical events, but to a large extent, it just seems that if you die some unnatural death, you've got a decent chance of being a ghost in the Warcraft cosmos.
But setting aside these figures, we then move on to the work of necromancers.
Necromancy appears, according to our new cosmos chart, to be its own branch of magic. Previously, we've seen necromancy performed by people who we might expect to use Fel magic (Warlocks) or Arcane magic (mages-turned-necromancers like Kel'thuzad.) That said, now that we're really locking down a lot of this stuff, it might simply be that these practitioners are actually branching out into this other type of magic.
We've encountered a few figures who claim to have dominion over death. Yogg-Saron is the self-proclaimed God of Death, but as an Old God, he/she/it is fundamentally linked to Shadow/Void magic, which our new chart considers distinct (and while the two are next to each other on the chart, Holy and Fel magic area also next to each other, so they don't have to be terribly closely linked.)
The Darkspear worship a loa named Bwonsamdi (among others) who, a bit like his real-world equivalent, Baron Samedi, is a death god. Yet given that Bwonsamdi is a Loa, that would theoretically put him in the domain of Life. It's probably a lot more complicated than that, what with Life and Death being opposing but also complementary forces (e.g. killing an animal to eat it means death for it and life for you.)
But if we're going to talk about necromancy, and beings of god-like power, we really can't avoid talking about the Lich King.
The Lich King commands the Scourge, and has become so powerful that we can't really get rid of him, lest the Scourge go on an uncontrolled rampage and spread unchecked.
Which is funny, because the Lich King is incredibly recent. For a being that has dominion over an entire sixth of the Great Powers in the Warcraft Cosmos, the Lich King has only just barely come to be. After the Second War, Ner'zhul opened several Dark Portal-like structures around Draenor to escape Kil'jaeden. Not only did his plan wind up breaking apart Draenor, but it didn't even work. The moment he stepped through, he was captured. His body was torn apart and his soul was attached to a set of armor - that armor (most importantly the helmet) would come to really be the Lich King itself. Arthas and later Bolvar would put that helmet on and thus become the new Lich King.
But here's some stuff that maybe doesn't add up. We know that Ner'zhul had access to Void Magic thanks to K'ara - a darkened Naaru. We don't know quite how much he used this in our timeline, but at least on Draenor B he was lousy with the stuff.
Yet he seemed to use this Void Magic to delve into necromancy, raising ghosts and skeletons to do his bidding.
Kil'jaeden was the one who created the Lich King, but how did he, a demon that used Fel magic by his very nature, wind up creating this font of necromancy?
Sure, it might be that Kil'jaeden's more versatile than that or that the necromancy came out of Ner'zhul's experimentations with the Void (perhaps even reverse engineering necromancy from Dark Shamanism, which seems to use the four physical elements and substitutes "Decay" in for "Spirit.")
But let's entertain another interesting notion: That the Lich King existed long before Kil'jaeden "made" him.
We don't really know what kind of magic Kil'jaeden performed to turn Ner'zhul into the Lich King. But suppose that the Dead - the Great Power of Death - has always had its own Titan/Naaru/Demon equivalent. But because death is, by its nature, kind of formless, this Essence of Death never really had an incarnation. Things would simply die, and perhaps some people would reach back into Death and pull things up into Undeath, but there wasn't a specific deity or conscious entity to invoke.
Perhaps Kil'jaeden was experimenting far outside of his ordinary wheelhouse and either created a vessel for this Great Power of Death to command, or unwittingly allowed Death to manifest as a figure of tremendous power - a figure that Kil'jaeden was unable to destroy once the Lich King turned on him.
Demons are kind of bad enough on their own, but what if Kil'jaeden unwittingly unleashed something far more dangerous than himself? The Lich King might only be about five years old (lorewise,) but already he's in a position where we just can't get rid of him.
The Lich King isn't really Ner'zhul or Arthas - we discover (SPOILERS) in the Frost Death Knight artifact weapon quest that Ner'zhul - long thought to have been utterly consumed by Arthas - is trying to use the soul of the Fallen Prince to wrest control of the shards of Frostmourne. Bolvar Fordragon now hold the position of Lich King, but if Ner'zhul is trapped in Frostmourne, along with Arthas, than that suggests that the Lich King - as an entity with enough of a consciousness to transform Bolvar's personality (while not aggressive or actively seeking the end of all life, Bolvar's Lich King is not quite the noble spirit that the old Regent was in life) - is somehow separate from everyone who has inhabited that role.
I'm very curious to find out more about the Shadowlands, to which I suspect the Lich King has unrivaled access. Whether a newcomer to the cosmic poker game or actually just as ancient (if not more) than most of the players, we really need to see what part the Great Power of Death will play in the future of Warcraft.