Undead (Kings of War)
The Undead of Kings of War are a disparate group that represent a lot of undead tropes and motifs found in fantasy. A Vampire Counts player could easily find a home for their units in the army, but the list actually has more units then it's counterpart in Warhammer Fantasy Battles did. This allows a wide variety of undead forces to be fielded.
Necromancy in Mantica has an interesting history. It predates the shattering of the Fenulian Mirror, yet is said to be a dark art overseen by the Wicked One Durunjak. It might be that vampirism was created by Durunjak, while more basic necromancy predates the arrival of the Celestians. Regardless, necromancy was created by the Ahmunites that would later split into the Empire of Dust and Ophidia.
In Ophidia necromancy is seen as a state art, regulated by the nobles and elite of society. There it is also a common sight to see both the living and the dead fighting side-by-side as if nothing were wrong. Given that it is also apparently common place to see demons and djinn walking the streets and bullshiting with the locals, maybe this isn't that odd after all. From here it is said that the dark art spread to Mantica and, presumably, to the lands of the far east.
The undead are despised and hunted by most nations outside of Ophidia, even more darkly aligned forces such as the Twilight Kindred and Abyssal Dwarfs are said to find their use repulsive. That being said, forces of the Abyss have often worked alongside necromancers to further their mutual goals, such as the alliance between Ba'el and Mortibras.
Notable Undead Warlords
A female vampire lady. She was featured on the cover of the first edition core rule book. She was a noble woman of one of the Successor Kingdoms known for being capricious, cruel, and vain in life. In undeath these traits got magnified. After becoming a vampire she slaughtered all of her kinsmen so that she had sole claim to the throne. Once solely in power, she had a great silver sword crafted that she wields in battle. Her goal now is the extinction of not only the living, but of every other vampire as well so as to leave her as the sole vampire in the world.
Her special rule is interesting to consider alongside her fluff. Called "The Promise of Love Eternal," it causes all enemy heroes who fight her in combat to suffer a -1 penalty to hit. This would seem to imply she offers vampirism and her love to opponents she faces, only to presumably then renege on the offer once the battle is at an end.
Mhorgoth the Faceless
The big bad of first edition, Mhorgoth the Faceless led a massive undead horde against Mantica in his quest to seize the Glade of Ways held by the Twilight Kindred. His chosen path there however, was a huge pain in the ass. Coming from east of Tragar, he invaded through the Free Dwarf Lands, slaying Thorrick Rockfist at the Battle of Ironhold Bridge before trying to cut through the Golden Horn of Basilea. He was driven back here, but is said to be plotting a return invasion.
Mhorgoth was originally a human born after the Winter War. Skilled in the magical arts, the Northern Kindred took him in to teach him the magical arts. He studied in Elvenholme for years, but it was discovered that he had secretly been practicing necromancy. Before he could be captured, he fled south. Hunted down, the elves cast a spell called the "Eternal Flames" upon him, burning the flesh from his bones. This didn't kill him though, prompting him to unleash a spell that killed everything within a mile of him. He vanished from history as this point, only to return two hundred years later in his invasion of the Free Dwarf Lands.
Mortibras the Necromancer
The villain of Dwarf King's Hold released in first edition, this campaign was reworked in Dungeon Saga as the "Dwarf King's Quest" module. Mortibras originally hailed from the Golden Horn and studied at the Order of the Ardent Light. As he studied, a voice was constantly calling to him. This voice eventually led him to leave Basilea in search of the body of Valandor. This led him to Therennia Adar where he discovered the body lying in repose there was a fake. Instead the body was hidden in Dolgarth, prompting the titular Dwarf King's Quest.
The efforts of the heroes in stopping Mortibras were only partially successful however. They stopped him from recovering Valandor's body, but he had seized the Tome of Valandor, allowing him to free the "voice" he had always heard, now revealed to be the imprisoned Ba'el. The heroes pursued him deeper into the ruins in the "Return of Valandor" quest module, but both he and Ba'el escaped the ruins and laid siege to Basilea in the Destiny of Kings supplement.
On the Tabletop
Tactics can be found here: Kings of War/Tactics/Undead Armies
As mentioned before, undead armies can be fielded in a massive amount of ways. They're good at hordes, good for elite cavalry and flier heavy lists, and they even have large infantry options. On top of that, they probably have the best model line Mantic Games has to offer right now.