Walach Harkon
Walach Harkon the Grand Master of the order of the Blood Dragon was a special character introduced way back in the 5th edition of Warhammer Fantasy. He takes his name both from Walachia, the province of Eastern Europe the real-life Vlad Tepes ruled over, and the Gaelic name Harkon meaning dark red (quite appropriate for a bloodsucker). According to his background, he was the first vampire sired by Abhorash after he himself was turned into a vampire by the Lahmian Queen 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 was 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 the OG necromancer 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 so impressed with the pair's raw martial skill that he turned them into vampires and started to train them personally. After the two cousins completed their training Abhorash decided that not putting their skills to use was a waste and so he went 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 of his own little Vampire Kingdom called Strygos. Wanting to make a sort of Vampire Utopia there Ushoran invited many vampires including Abhorash and company to live there and they promptly accepted his offer. For a time things were going pretty well, but then Neferata finally caught wind of what Ushoran had done and 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 surviving inhabitants of the kingdom were left adrift and wandered about aimlessly until one day a major event happened that would change the course of Abhorash’s bloodline. One day while traveling through the mountains Abhorash accidentally stumbled across a red dragon who took exception to the invasion of his territory. Not one to back down from a fight Abhorash engaged the beast in solo combat and after a fierce fight he slew it and drank its blood. To his astonishment Abhorash discovered that after this last drink he no longer felt the need to consume blood anymore. Understandably Abhorash immediately raced to tell the rest of his bloodline of his transformation and once he did so he charged them with seeking out the most worthy opponents so that one day they would be skilled enough to face a dragon and be cured like he was. His lineage agreed to this and then decided the best way to accomplish this was to change their names and then 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. Feeling the need for a good fight after so long without one Walach knocked on the front gate and challenged the entire order to combat. Stupidly the Knights accepted the challenge and Walach promptly proceeded to kill their grandmaster with his Crimson blade and beat the rest of the knights senseless 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 newly renamed Order of the Blood Dragons. To celebrate his new job Walach and the Blood Dragons promptly turned on the people they had once defended. They fed on passing travelers and the people who lived near the Blood Keep, turned the local women into vampire brides who would bathe in blood to maintain their looks, and just generally made nuisances of themselves. This turned out not to be such a good idea on their part though as after a few years of this Walach's actions were uncovered. In response the Witch Hunter Gunther van Hel gathered an entire Imperial army together and 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 kill Harkon's vampire wife Aurora and his favorite Vampire “son” Mikael they failed to get the rest of the Blood Knights and Harkon who managed to escape. For a time the Blood knights were scattered and most traveled around the Old World fighting duels with random folks and acting as mercenaries. Walach meanwhile would gather together a small company of new blood knights and under the banner of Walach's favorite fallen "son" Mikael they would continue on their self appointed mission to seek out the strongest possible opponents they could find. Eventually, after a few years of this, enough time passed that Walach and his accompanying 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 new undead waifus and venturing out to fight any and all worthy opponents they could get their hands on, though this time the Blood Dragons tried to tone down the whole slaughter of the innocents thing and act more like honorable (if blood thirsty) knights so as to not draw as much attention to themselves.
The End Times
Things continued like this for a few years until the newly resurrected Nagash started recruiting many of the world's most powerful undead lords as 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 mystical Auric Bastion erected by the wizard 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 Emperor Franz 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 the blood god Khorne who apparently thought the Blood Dragons were stab happy enough to overlook their undead state...or the fact they can use Necromancy. This clusterfuck then proceeded to reach its peak when Harkon, riding an undead dragon, engaged in an aerial duel with Karl Franz, defeating him and casting him low.
Following the seeming 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 dragon from Harkon and then had the beast rip its former master in two, finally destroying the infamous Blood Dragon once and for all.
Stats
Sadly despite his awesome potential as a character Walach not only didn't get a model, he was barely mentioned in the fluff besides what was written above and only managed to get statted out way back in the very same edition he was introduced in (even End Times did not give him a new statline, the poor bastard). So yeah there are no official stats for him using the flying zombie dragon mount he used in his last battle and we don't know if Khorne granted him any sort of blessing.
If you wanted to do a homebrew version of End Times Walach you could modify a model of Zacharias The Everliving or some other Zombie Dragon rider and raise his default point cost to say 650 like most of the other Mortarch's or 795 since the Dragon normally costs 245 points on its own. Then give him all three pieces of otherwise optional magic gear listed below and add the Frenzy rule to represent his new allegiance to Khorne. Given his already heavy armor, you wouldn't have to necessarily have to add any specific anti-magical Khorne-related gear to buff him. As for spells...well Khorne normally hates magic but given Walach would still have to be a Necromancer to control his Zombie Dragon you could conceivably justify him still having magic by giving him only the spells Ryze - The Grave Call and Akar'aran the Dark Riders. It wouldn't be too much of a contradiction for Walach to have summoning spells given Khorne is ok with people using magic to summon his demons. Alternatively, you could swap out the latter spell for Invocation of Nehek to heal your undead units on a 6+ roll, but a healing spell doesn't really match flavor-wise for a Khorne follower...even a Khorne worshipping vampire...ugh.
Anyway, for his Zombie Dragon mount, it's normally listed as a 245 point monster unit but just ignore the point cost and use the followings stats from the 8th edition:
6 Movement, 4 Weapon Skill, 0 Ballistic skill, 6 Strength, 6 Toughness, 6 Wounds, an initiative of 2, 5 attack, leadership 4. As previously mentioned it's listed as a monster and comes with the following keywords: Fly, Large Target, Scaly Skin (5+), Terror, and Undead. As for Special abilities, its Pestilential Breath attack causes -3 to armor as well as causing an S2 penalty to anyone it hits. It also has Swarm of Flies (ew!) which causes -1 to To Hit rolls in melee and if you do decide to go with the Invocation spell you could use it to heal the dragon. Fair warning though while it can really wreck the day of any enemy heavy cavalries or rank and file, it will probably die to cannons or a Heroic Killing Blow. Still, a wonderful kick-ass model even if like most fun things, it will be a magnet for artillery and mass shooting.
If however, you wanted to play regular vanilla flavored Walach then according to the 5th edition of the Warhammer Fantasy game Walach's is listed as a 550 point heavily armored named special character. His default mount is an undead barded Nightmare horse of the non-winged variety which has the following stats:
8 Movement, 2 Weapon Skill, 0 Ballistic skill (obviously since it's just a fancy undead horse), 3 Strength, 3 Toughness, 1 Wound, an initiative of 2, 1 attack, 3 leadership and since it's specifically mentioned as being barded it adds a +1 armor save roll. Were it on its own it would be considered a 16 point war beast but that's not important since its point cost is already included with Walach's. What is important is that it's undead and so you can use the invocation spell to heal it.
As for Walach himself, he's got:
6 movement, 9 Weapon Skill, 6 Ballistic Skill, 7 strength, 6 toughness, 4 wounds, 9 initiative, 5 attacks, and leadership of 10, and thanks to his heavy armor he has an armor save roll of 4+ (3+ when mounted). In addition to this, he's a Vampire Lord and thus considered a level 3 Necromancer that comes with 2 spells from the lore of undeath. Bloodline ability-wise he comes with the following traits: Doomrider, Strength of Steel, Might of Arms, and Red Fury (any modifiers for these are already included in the above statline).
In addition to the above regular non-End Times Walach can come equipped with the following magical items in exchange for an increased point cost:
For 75 points extra, you can get the Crimson Blade which has a special ability determined by an extra dice roll. On a roll of 1 to 3 his opponent loses 1 attack in addition to whatever damage he would normally take. On a roll of 4 to 5 his opponent loses 1" of movement and his opponent is considered separated from their unit if they fall below their former unit's movement speed. Lastly, on a roll of 6, the Crimson Blade beheads and thus outright kills the unit it was being used against.
For an extra 25 points Walach gets his legendary Blood Chalice which he can use at the beginning of each undead turn to add one of the following abilities at a time: It can heal 1 wound point suffered in battle but does not raise his max wound stat above normal, it can raise his attack stat to 6 until the start of the next undead turn, or finally, it can transform his sword into a flaming weapon that deals two wounds instead of one and lasts until the start of the next undead turn.
Finally, for an extra 50 points Walach gets the Blood Dragon Standard, a magical Standard that fills all undead creatures within a 12" radius of it with HATRED for any living creatures and allows the undead to reroll its To Hit roll for the first round of the first hand to hand combat action it fights against such beings. Subsequent attack rolls are rolled as normal.