Walach Harkon
Walach Harkon is the Grand Master of the order of the Blood Dragon the first vampire sired by Abhorash after he himself was turned into a vampire by Neferata. In addition to being an all-around martial badass, who rode into battle wielding the infamous Crimson blade and a chalice filled with the blood of his strongest opponents he's also well known for being cousins with a certain batshit crazy vampire Pirate Captain by the name of Luthor Harkon.
The Legend
Originally he was known as Walak of the Harkoni tribe and lived with his cousin Lutr in Nehekara back before Nagash and his botched spell of super resurrection turned the place into a miserable wasteland. One day he and his cousin happened to catch the attention of Abhorash who was impressed with their martial skill and so they were promptly turned into vampires and after training them further in combat he gave them jobs in the city of Lahmia; Lutr was given command of the city's harbor guard and Walak was made master of the palace guard. When the city fell following the revelation of Neferata's nature as a vampire Abhorash, Walak and Lutr managed to escape and wandered about for a bit until another vampire Ushoran went and made himself king own little Vampire Kingdom called Strygos and invited the three of them to live there. They promptly did so and things were going pretty well until Neferata decided to throw a bitch fit over the revelation that there was a vampire kingdom she was not in charge of out there. In response to this slight she convinced a bunch of asshole barbarian humans to attack the kingdom and while Walak and the other vampires managed to chase them off the distraction allowed a horde of Orcs to flank the exhausted defenders and they promptly proceeded to wreck the place. Following this disaster, the various vampires decided to change their names and go their own separate ways. In the newly renamed Walach Harkon's case after many centuries of wandering his meandering way took him to a place called the Blood Keep, a bastion of The Empire which guarded the area around the pass into Bretonnia and was manned by an order of knights called the Blood Dragons. Feeling the need for a good fight after so long without one Walach promptly knocked on the front gate and challenged the entire order to combat. Stupidly the Knights accepted the challenge and Walach promptly beat them all before turning the stronger members of the Order into vampires and the weaker ones into Wights. From that day seven centuries ago till the End Times Walach Harkon was undisputed Grand Master of the Order of the Blood Dragons and the lands around Blood Keep would be his prey. However, Walach's actions did not stay unnoticed and eventually the Witch Hunter Gunther van Hel caught wind of what was going on. Not liking any of that Vampire nonsense Van Hel and an entire Imperial army launched a three-year-long siege on the Order's headquarters in an attempt to destroy them. Although the Imperials eventually managed to break into the fortress and killed Harkon's vampire wife Aurora and his favorite Vampire get Mikael they failed to get the rest of the Blood Knights and Harkon who managed to escape. For a time the Blood knights traveled around the Old World as mercenaries with the company bearing the banner of Walach's favorite "son" Mikael as a way to remind the order of who they were and keep the fires of vengeance burning in their undead hearts. After a few years of mercenary work, enough time passed that Walach and a few of his blood knights decided that things had probably died down enough in the Empire that they could head back home without issue and so they promptly did just that. They rebuilt their fortress manned the walls with new wight guards and then promptly settled down to an unlife of carousing with their undead waifus and fighting any and all worthy opponents they could get their hands on.
The End Times
They did this for few years until the newly resurrected Nagash started recruiting many of the world's most powerful undead lords to form his generals. Harkon and his now thoroughly insane cousin Luthor heard of this and decided to join up and became two of Nagash's Mortarchs. Now with a sweet new title, Walach was assigned to help Vlad von Carstein protect the Auric Bastion erected by Balthazar Gelt in the northern Empire. Under the command of Vlad von Carstein, Walach fought at the Battle of Alderfen. Here a Nurgle force breached the bastion and started butchering the Imperial forces beyond it. All looked lost until the surprise undead counter charge, led by Harkon and some of his knights, halted the Chaos advance. However, in his zeal Harkon and his knights made a major blunder and kept pursuing the Chaos forces through the breach which caused them to be trapped behind the bastion when the wall was finally resealed.
It wasn't until the Battle of Heffengen that Harkon would resurface. After the total collapse of the Bastion, Vlad von Carstein sought an alliance with the Empire and to this end planned to fight alongside the Imperial forces at Heffengen to hold back the Chaos hordes led by Crom the Conqueror. Just as Vlad was about to reveal himself and be recognized by the Emperor as a hero and ally Harkon and his knights unexpectedly charged onto the battlefield from the Chaos lines and started slaughtering the Imperials. Turns out that while trapped behind the Bastion, Harkon had fallen to temptation and pledged himself to Khorne who apparently thought the Blood Dragons were killy enough to overlook their undead state...or the fact they can use Necromancy. This clusterfuck then proceeded to reach its peak when Harkon, riding a skeletal dragon, engaged in an aerial duel with Karl Franz, defeating him and casting him low.
With the apparent death of the Emperor the Imperial forces retreated and the battle was lost. However with the armies of the Empire now unwilling to accept the undead as potential allies Vlad's plan was ruined and the now righteously pissed-off vampire decided he would not let Walak's action go unpunished. When Harkon swept from the sky and tried to pin von Carstein to the ground, Vlad used his superior magical abilities to wrest control of the skeletal dragon from Harkon and then had the beast rip its former master in two, finally destroying the infamous Blood Dragon once and for all.