The Necrarchs are a Bloodline of Vampire Counts who are notable for worshiping Nagash and being focused on inventing new spells and other magical shit. They are basically the "Wizard Vampires" of Warhammer Fantasy.
The Necrarchs descend from W'Soran, the only one of the first vampires who was genuinely loyal to Nagash. When the Father of Necromancy fell in battle, W'Soran and his acolytes gathered as much of Nagash's lore and studies as they could and fled Nehekhara. W'Soran became consumed with the vision of the whole world delivered into undeath, where his children would rule for all eternity over an unliving planet, but knew that the need for living blood made this an impossibility - his experiments in learning to subsist on Dark Magic over blood were partially successful, but cursed the bloodline to hideous forms and broken minds.
W'Soran was betrayed and killed by his foremost apprentice, Melkior, after which the Necrarchs scattered across the world, unable to control their paranoia any longer. They have since become a race of scattered schemers and mad dreamers, working towards the day when W'Soran's vision will become reality.
Necrarchs are unusually adept at magic, in no small part due to their love of actively studying how to use necromancy, in comparison to other vampire Bloodlines, who tend to rely on their instinctual affinity for necromancy spells. However, perhaps due to this, Necrarchs suffer an unusual level of physical degeneration; whilst still stronger, tougher and faster compared to a normal human, they are outdone in all of these capacities by the other Bloodlines - way back in 6th edition, the drawbacks for taking the Necrarch Bloodline were that your vampire couldn't use mundane armor or weapons, and suffered a -2 to Weapon Skill, in exchange for their +1 bonus to casting rolls and Thralls gaining the ability to use Arcane items.
Related to this, Necrarchs are also one of the two most hideous Bloodlines of the Vampire Counts, next to the Strigoi. Although immortal, a Necrarch's body withers and decays, reducing them to gaunt, almost lich-like figures of leathery skin stretched taut over frail bone, shrouded by the stink of decay and rot. Of all the vampires, Necrarchs need to feed the least frequently, and often conduct experiments seeking to free themselves from bloodthirst altogether.
The Witchsight - the ability to perceive the Winds of Magic - is overwhelmingly strong for a Necrarch vampire. The material world in which they inhabit is a misty, foggy, unreal-seeming place to their eyes, whilst the spirits of the dead, corpses, and places abounding in Death Magic are all crystal clear and realer-than-real to them. This only adds to their propensity to seem, if not be, raving mad.
The Necrarchs are characterized by an unusual fixation on the notion of fate and prophecy, and by a level of paranoia considered out-of-hand even by other vampires.
The Necrarchs are the most isolationist of all the vampires. With little need for blood, and gripped by a Bloodline-wide sense of paranoia and possessive jealousy, they shun the company of all others, creating new Necrarchs rarely and reluctantly. A Necrarch typically dwells in a forbidding tower or crypt somewhere in the deep wilderness, surrounded by the undead; the only living beings willing to coexist with Necrarchs are usually mutants, madmen, outcasts or necromancers. They look down on the other Bloodlines, and trust them even less than they do each other. They believe that their kinsfolk will be destroyed when the world undergoes its "rebirth", since they are too foolish to learn to subsist without blood like the Necrarchs do.
Necrarchs at War
Necrarch legions are typically much larger than those of other Vampire Counts, since the Necrarchs are more adept at raising the dead. They heavily bolster their forces with necromantic constructs or the more powerful breeds of undead monster, such as Zombie Dragons and Terrorgheists, due to that self-same greater affinity for studied and controlled magic.