Dieter Helsnicht, the Doom Lord of Middenheim, is a villainous necromancer from Warhammer Fantasy Battles. Featured prominently in the 4th edition Undead armybook and in The End Times, where he was one of the lieutenants of Nagash.
He was once a great and much feared wizard who was forced to flee from the city of Middenheim when it was discovered that he was a Necromancer. He built a secret castle in the Forest of Shadows and concealed it so well that it was never discovered by the Witch Hunters who searched for him for years. There he slowly built up his strength and plotted his revenge.
Dieter Helsnicht was once a great and renowned wizard who lived in the Empire city of Middenheim during the Time of the Three Emperors. At that point, Middenheim was a haven for wizards of many kinds from the persecution they suffered in other parts of the Empire. Over the course of his research, Dieter learned of the great Necromancer king Kadon and decided to travel to the lands that he had once ruled in the Border Princes in order to find out more about this enigmatic and evil figure. It was while he was there that Dieter first started to hear rumours of the return of Nagash after his defeat by Sigmar. Dieter, drawn by an irresistible curiosity, continued his journey and travelled to the legendary fortress of Nagashizzar. What happened to him in that cursed place is not known, but he returned to Middenheim a completely changed man, his hair turned prematurely grey and his skin tinged with an unhealthy pallor. The Doomlord is born! Shortly after his return word began to spread of evil practices and vile rituals being performed in the dead of night by Dieter and his henchmen.
Only too aware of where such things could lead, the High Priest of Ulric gathered a company of Knights and descended on Dieter's dwelling. They arrived just in time, disrupting a magic ritual that would have allowed Dieter to raise the dead buried in Middenheim into a powerful Undead army. Shaking his fist and vowing revenge Dieter fled from the city, swooping away over the heads of the astonished High Priest and Knights atop the back of a monstrous manticore. Dieter was a man of deep, if twisted, intelligence and had prepared for every eventuality. He had built a secret castle deep in the Forest of Shadows to which he could escape should his activities be discovered.
After decades of preparation his evil plans reached fruition, and the Doomlord marched forth at the head of a huge army of undead monsters to attack Middenheim. Thus he intended to bring death to the city of his birth. After a long campaign and many victories he was finally stopped at the Battle of Beeckerhoven. The Doom Lord's body, however, was never found and it is commonly supposed on the back of the undead manticore he rides in battle. As the Doomlord, Dieter has an iron will and can bend even the most powerful creatures to his authority.
With the return of Nagash during the End Times, Helsnicht was recruited as one of the Mortarchs and accompanied Nagash's armies into Nehekhara. Joining the central force led by Krell, Helsnicht's necromantic power, paired with the tactical brilliance and sheer brutality of the wight warlord, proved to be nigh-unstoppable. Flying atop a bat winged horror (almost certainly the reanimated remains of his Manticore) Helsnicht was instrumental in the running battles between Krell's forces and those of King Phar of Numas. He bolstered Krell's forces by creating several Morghasts, but also revealed a disgusting new quirk in his madness, he had taken the habit of devouring the brains of lesser Necromancers, seeking to absorb a portion their knowledge.
When the united armies of Nagash met those of Settra at the gates of Khemri, Nagash's true plan for Helsnicht was revealed. Nagash entered the Nehekharan underworld with Helsnicht and had him cast a great ritual while he battled Usirian, god of that dark place. Boosted by the knowledge and power of his master, Helsnicht's ritual bound the entirety of the spirit realm to himself, which he then sent to overwhelm the faceless-god. Crippled and beaten, Usirian was consumed by Nagash, who finally was restored to his full strength. Helsnicht would ultimately remain in the Nehekharan underworld, becoming its new guardian.
However, Helsnicht is yet to be seen in the realms of Age of Sigmar and his current status is unknown.