The Necrarch were a cabal of vampires focused primarily on the mastery of necromancy.
The vampire scholar W'soran was the first of the Necrarchs. First during his time in Lahmia, and later in his time in Nagashizzar and later still in Mourkain, W'soran would gather mystically gifted proteges and teach them the secrets of necromancy. He used these acolytes as servants and as research material if they displeased him. After W'soran's eventual destruction these powerful undead sorcerers spread across the world.
Necrarch vampires are hideous in appearance as their constant exposure to death magics twists and corrupts them. Interestingly though there is an advantage to this; Necrarch vampires are much less afflicted by the blood lust of other vampires, with some able to forgo blood entirely, using the dark magic to sustain themselves.
Though rare, Necrarchs are much sought after by young necromancers looking for a tutor.
In previous versions of the game the 'Necrarch' were a bloodline, which is to say all Necrarchs were a descendant of W'soran. In the modern version it seems any vampire with a specific interest and obsession with necromancy is a Necrarch.