The founder and usurper of the Necrarch Bloodline of the Vampire Counts. Both were huge fucking nerds with a penchant for playing army men as adults. Note that there are two different versions of the end of the story of W'soran/Melkhior. It's more up to personal interpretation which is canon, although the former story is supported by the legend of Ushoran leading to it being taken as slightly more canon.
He seized the opportunity to do what he had always wanted to do and...promptly sealed himself up in the Great Library of Nehekhara to read. Forever.
He turned some minions who shared his interest in avoiding social contact, but otherwise just sat reading until Alcadizaar laid siege to the city, where he left the confines of the library and raised a fuckload of Undead for Abhorash to lead. He then went right back to reading, not realizing or caring what was happening until the moment the Great Library was on fire.
Many of his followers stayed in the building, but he and the ones without WIS as a dump stat loaded up on as many books as they could and hit the road. As the library burned behind them the dreams of Tomb Kings players taking Forbidden Lore, as well as humans being able to claim they figured out magic without Keeb help, went up in smoke.
The remaining Vampires united under Nagash and fought back against the humans, but meddling Skaven caused them to lose. While the Bloodline leaders fought amongst each other, W'soran hid in Nagash's library and contentedly read. When the others fled, W'soran stayed with Nagash and rode in his own chariot doing the Level 4 Wizard thing. As a reward, Nagash gave him an autographed copy of one of his nine books series "Necromancy and You" as well as a Black Pyramid tote bag. During this time W'soran often butted heads with Nagash's other Level 4 Wizard Arkhan. The former thought the latter was cowardly, the latter thought the former was cocky. They two often argued; when they weren't disagreeing on the best way to conduct the battles, they bickered over which of them was the better wizard, which form of undeath was best or who was Nagash's favorite.
Here, the story of W'soran and Melkhior splits. The W'soran version follows.
After Nagash was killed W'soran turned more nerds into Vampires and crossed the impassible mountains to join Ushoran, who had been kind of a bro to him. He aided Ushorsn in building up the power of Strigos, and helped Ushoran claim the Crown of Nagash. An afterthought came that Nagash wouldn't have been to pleased about that and that maybe the Crown of Sorcery was better left in the hands of an actual sorcerer (himself). This inspired W'soran to began to experiment with imbuing one's own essence into physical objects, skirting along the magical theory of elf Soulstones but instead creating the process of Lich phylacteries. He eventually created a helmet of his own and placed part of his soul within it, then gave it to a tribe of humans called the Dresca and watched them tear themselves apart through his subtle mental influence. He made a pact with the leader of a human tribe named Vorag Bloodytooth, who in exchange for helping make him powerful built W'soran a fantastic and secret laboratory fortress. Here W'soran conducted many EEEEEEEVIL experiments and perfected the Lore of Vampires. During the fall of Strygos, W'soran and his followers ambushed the Strigoi while they were fighting greenskins. Eventually, W'soran faced Ushoran alone and lost due to the influence of Nagash through the Crown. His forces were crushed by the Strigoi, and he himself fled through the burning city causing as much damage as he could to slow down the pissed off Strigoi, Strygos, his own traitorous followers, and Orc pursuers. During his retreat he found a large vein of Warpstone and consumed it, using the boost in magic to collapse a mountain which killed many of those chasing him and cut off the rest.
He used his magic to hastily construct a mountain keep that could only be reached by a long mountain pass filled with traps and monsters, then awaited his enemies. The Lahmians and Strigoi made it to the pass, but most were slain between battling each other and falling into traps laid by the mad scientist Vampire. Melkhior, clever and knowledgeable about his master's preference for traps, reached the keep and pretended to warn his former master about the coming threat, and convinced him they would make a final stand inside W'soran's own vault. W'soran found himself fighting the enemy Vampires alone, and after slaying them to the last Melkhior attacked his master from behind. Melkhior taunted him as W'soran lay dying, and W'soran seemingly fought his own dying breath to make out a few last words to his traitorous pupil. As Melkhior leaned in closer with a grin to hear what the old Vampire had to say, W'soran seized him and growled with a wicked smile that he had discovered the secret of true immortality. W'soran bit his student in the neck, and his soul entered the body of Melkhior and consumed the younger Vampire's soul and knowledge. When the Lahmian reinforcements finally arrived at the keep, they found the corpse of W'soran and the (un)living student claiming to have done the deed. He told them he wanted to join them, and would give them all the secrets of W'soran provided he had time to translate the many tomes and decipher the experiments and tablets within the vault. He fed them a steady stream of lesser magics and artifacts before returning to his wayward students, gathering them into enclaves dedicated to advancing different areas of necromantic and alchemical study. Each knows little of the other enclaves, and none know all but himself. He himself is the master of the large and isolated Necrarch Bloodline, and over time and wearing different guises has ingratiated himself one way or the other with all of the Vampire Bloodlines.
While each different Bloodline may claim to control an army, claim to control a nation, or claim to be invincible it is W'soran and the Necrarchs who's combined strength is represented by the application of Undead monsters, unbelievably powerful and previously unknown magic, and subtle manipulation to the combined might of the others.
The above story represents the basic interpretation of the story. Another version exists within the fluff.
In this one, after the fall of Nagash the Necrarchs built their power and spread throughout the world without the parts involving Ushoran and the Strigoi. W'soran, being more trusting in this interpretation, took loyal students with him as they traveled in secret administrating to the Necrarch enclaves. At one point, Melkhior began to read the books of Nagash while W'soran meditated or conducted intensive research. During one of these meditations, Melkhior slew the body of his master, trapping the soul of W'soran in the spirit world where he gathers armies of Ethereal undead to prey upon the living and eventually his former student.
Melkhior built a magically hidden tower in the Great Forest which is found within the Empire, and studied magic alone.
Zacharias The Everliving
The following story goes along with the latter story where Melkhior defeats W'soran, but for the sake of unified canon (as in the current canon both W'soran aiding Ushoran built his empire and this story appear to both be true) one can assume that a different version of is happens in the former story as well.
At one point a thief named Zacharias attempted to breach the tower where Melkhior was dwelling and steal a book of Nagash and, impressed that the young man had foiled his magical defenses, Melkhior made Zacharias his pupil. As the years went on, Melkhior became more and more insane and would go on Strigoi-like rampages to drain whole cities of blood. Zacharias took advantage of these times to read the book, and when Melkhior returned one night and caught him at the act Zacharias fled.
Wounded by the pursuing minions of Melkhior, hid in a cave and fell into a restorative hibernation for ten years. When he awoke he found that a Black Dragon had begun to use the cave as a lair. He promptly drained it's blood over a very long time (since nothing can wake up if a vampire feeds on it while it's sleeping) which caused him to grow in strength and cured his Vampiric weaknesses and need to feed on blood. He promptly reanimated the Dragon's corpse as a mount, then gathered an army and returned to lay siege to the tower. The two battled, and from here the story splits once again. In one version of the story, Melkhior only narrowly survived the confrontation and fled to plot revenge. In the other, he perished and Zacharias took over as head of the Necrarch Bloodline and organized them into an effective force preparing to attempt to conquer the Empire in a manner similar to the von Carsteins. Either way, Melkhior's treasures and book of Nagash wind up in the hands of Zacharias.
When Nagash came back, Nagash contacted him for his service, but found that Zacharias thought he was his equal. For this insult, Nagash promptly set his brain on fire in his head and instantly killed him. Unfortunately, this and a complete lack of any mention of W'soran or Melkhior in End Times suggests he really was killed by Zacharias, although no mention of the other Necrarchs were made either. Unfortunately, End Times left out quite a few things and the lore answers that Josh Reynolds provided were rendered non-canon by usual Games Workshop douchebaggery so either the Necrarchs were all wiped out alongside Zacharias, completely offscreen or just joined Nagash/went into hiding equally offscreen, and either way were 100% destroyed with the old setting when Age of Sigmar came to be.
Like the other spellcasters of the game (Empire Mages, Tzeentchian Warriors of Chaos, Saphery Mages, and the like the Necrarchs are more likely to be found in the company of other Bloodlines than in an army of their own. Should have need to however, Necrarchs are very adaptable.
Any option you want to take can be handwaved as another Bloodline recruited to their service, although particularly "Necrarch" options include Zombies and Skeletons for Core raised in bulk as needed (failed test subjects in mass graves is another good explanation), Corpse Carts (as gatherers of new specimens) and Hexwraiths/Spirit Hosts (vengeful test subjects, experiments in soul-binding, or the minions of W'soran in the story where his soul is doomed to wander) as Special, Black Coaches (as a protective form of Chariot for the studious Necrarchs to direct battle from) and Mortis Engines (an arcane object created by Necrarchs powering their creations) for Rare. For Lords and Heroes, no named character is fitting (apart from possibly Mannfred the Acolyte. After all, he has studied everywhere, probably including under a Necrarch tutor at some point) unless one wants to use the stats of one of the Mannfreds to use as a Necrarch model. Necromancers are a possible option as an assistant, but really any options should be spellcasting Vampires and Master Vampires. Use of any other Lores other than Lore of Vampires is within theme if you choose to use them.
On the Tabletop
W'soran/Melkhior once had models and stats, but have long since been removed from the game and discontinued. If you can obtain the models, one can use W'soran or Melkhior (depending on which version of the fluff you consider canon) as a Master Vampire spellcaster in the Lore of Vampires riding an Abyssal Terror.
Necrarchs used to be one of the more popular Bloodlines until von Carsteins became the only Bloodline of any importance in the game in 7e, and as a result have a fair number of on foot and mounted models. All are out of production, so eBay is your only source.
As for how they used to be in their old rules, Melkhoir knocks in at 660 pts , he causes terror (with -1 to enemy leadership, but the modifier doesn't affect undead or daemons naturally), he is immune to all psychology, he is a lvl 4 wizard that gets one more spell than usual, has stupidity, has an abyssal terror and most importantly he can recast a spell on a 3+ (you still have to cast it as normal after passing the recast roll), though he cannot cast the same spell twice on the same unit. Melkhoir also has three very special items that are awesome, he's got a magical sword that gives the models he hits -1 to their to hit rolls (he doesn't need to wound and the effects are not cumulative), he has a book he wrote that allows him to automatically cast a spell (it doesn't even use up any of your magic power either), and the spell that he casts can be recast as normal, but after using it, roll a die, on a 1-2 it can't be used again for the rest of the battle, and he has a black cloak that makes it impossible to shoot him. Literally, he cannot be shot at or hurt by shooting ever, not even by warmachines (he takes no damage if he gets hit) and not even by magical missiles (again he takes no damage) which is awesome combined with the sword so he doesn't get hit in combat, and combined with the book so he can keep shooting out spells. Statwise he's a Necrarch Vampire Lord, so also good in combat.