At the moment the Platinum Dragons were born, one of them created the Throne of the Lich. This altar transformed every body around it at that time into various different Undead species. Many of which would be able to be created by the strongest Undead, like Skeletons, Ghouls and Banshees. Others could not be created that easily, or at all, like Liches and Val'Kyr. All Undead except for Val'Kyr can't leave the presence of a Lich for too long. If they do they start to turn to dust, permanently.
The interesting thing about the Undead is, that any species can become any sort of Undead. These are the different sorts, ranked by power.
The Order of the Liches leads the Undead armies. Two factions that are part of the Undead kingdom are the Cult of the Damned and the Dark Riders. Another separate faction would be the Forsaken, which is not part of the main Undead force.
The Liches were the strongest of the Undead, being able to conjure Skeletons and Ghouls at will as mindless slaves, for this reason, they have always led the Undead. Most Liches are part of the Order of the Liches.Â
A Lich looks a lot like they used to when they were still alive. Their skin tends to be grayer, eyes sunken, loss of hair. All around, they look like a walking corpse, but don't underestimate their power. They have the ability to float and can summon and command a whole army of Undead minions.
Liches are created with the spell Create Lich.
The Val'Kyr are spectral beings and among the very first Undead created. They are so unique, that no one has yet succeeded in making new ones. All the Val'Kyr are sentient beings, and they are the only ones that can raise minions that aren't mindless, unlike the Liches.
Val'Kyr appear as ghost-like winged humanoids, similar to Aasimar, but are really neither, as they are quite corporal. Their skin glows a faint blue, and they have large feathered wings, which they don't use as they can just float around too.
Banshees and Wraiths are incorporeal beings, with one being female and spectral, while the other is shadowy and male, respectively. Liches can't make these beings without help, they require a short and simple ritual. Most Banshees and Wraiths aligned themselves with Kal'Thozul, which resulted in them now being bound to the Cult of the Damned. They aren't exclusive to the cult though.
Banshees and Wraiths are created with Greater Create Undead, which can only be cast by Val'Kyr.
Ghouls and Skeletons exist both as both mindless and sentient Undead, depending on who raised the individual. When a Lich or Necromancer raises them they are mindless slaves without a name. But when a Val'Kyr raises them they remember their name and identity.
Non-sentient Ghouls and Skeletons are created with Animate Dead. Sentient ones are created with Create Undead, which can only be cast by Val'Kyr.
Abominations are mindless constructs crafted from the body parts of dead humanoid beings. They are always controlled by their creator.