Abhartach. Information on just how real this person was is sketchy, but that’s not entirely surprising.
Record-keeping from the 5th and 6th centuries leaves just a smidge to be desired. We do know that Ireland was once ruled by a scattering of chieftains, who staked claim to lands and held their territories against intrusion from neighboring chieftains. In an area that’s now the northern Derry, one of the chieftains was called Abhartach. No one liked him, as he was reportedly incredibly evil, and rumored to have some sort of magical powers. The magic part might not be legit, but we can believe in someone so downright nasty, he managed to ally all his neighbors against him.
He was finally killed by a rival chieftain called Cathan, who buried him standing upright in his grave. The legends say he returned the next night, and that he demanded his subjects pay him an entirely new type of tribute: a bowl of blood. Locals appealed to the monks at Gortnamoyagh, who told them he was a particular sort of undead called the dearg-diulai, who drank blood and were impossible to kill. Instead, they needed to return him to a grave that he couldn’t escape from. So they tracked him down, killed him with a wooden sword, and re-buried him upside down. After adding ash twigs and thorns to the grave, it was sealed with a massive stone that’s still there today. Most call it the Giant’s Grave, but the ancient name is Leacht Abhartach, or “Abhartach’s Sepulture.”
How real is the story? We’re not sure — witnesses are hard to come by these days. But we do know that, even today, the land is supposedly still cursed. Attempts to clear it ended badly, and it’s been at the center of all sorts of disagreements between the family that owns it. Is it a vampire’s curse? We’d rather not find out, by the sounds of him.