Millard Dynam Stoop - as of late known as Lord Moldypants and previously anti-known as You-Probably-Don't-Know-Who - is a powerful dark wizard and a would-be Dark Lord of questionable competence. He's presumably some kind of an ultimate nemesis to the hidden wizarding world represented by such parties as the Ministry of Ministering and the Hoggelrynth school. His worst enemy personally is Torg Potter, or would be if he didn't have his eyes on the wrong Torg.
The young Millard Stoop attended Hoggelrynth and showed exceptional talent, earning the title of Wizard-Supreme in the Try-Gizzard Tournament. Even then he was evil and snake-like by appearance. There appears to have been a female relative of his, Zeral Stoop, going to the school at the same time, but nothing is known of her. At some point, Stoop also encountered Bundemort Wunnybun's familiar, the bunny genie, and managed to gain a degree of control over it after managing to make a wish that the genie couldn't horribly misunderstand on purpose. When a graduate student, he had his feet fondled by Gandledorf due to an unfortunate magical anagram.
Stoop began to build a following, and at some point (perhaps shortly before the year 1980, judging from the fact that Torg Potter is about the same age as Torg), for unknown reasons, he attacked the infant Torg Potter and his parents. His intention was to use a combination of curses to kill the parents and wipe them from history - which seems to have succeeded - and to turn the baby into "something small, weak, forgetful who wets himself and easily spreads common cold germs to hapless people". As Torg (not Potter) later pointed out, this wouldn't have had much of an effect on a baby, but it did have an effect when the magic somehow rebounded back to Stoop himself. Some part of the curses intended for the parents must also have affected Stoop himself, as he was also wiped from historical records and people's memories.
Some evidence of Stoop's existence remained, and people were aware that there had been someone, whoever the heck he was, who was attributed having done certain things. Even those encountering and interacting with Stoop had difficulty remembering who he was. Thus, he was known to everyone only as You-Probably-Don't-Know-Who (or "you probably don't know who"), which was appropriately mysteeeerious. Of his servants, only Lucius Malfoy remembered enough to know that there had been someone whose return should be effected.
In 2003, Lucius Malfoy arranged for Homogenize Milktoast to gain possession of the book How to be Possessed by You-Probably-Don't-Know-Who (or the other way around). This caused havoc at the school as the possessed Homogenize had people summon Wunnybun's genie familiar, which interpreted everything they said as a wish to turn them into chocolate. It was thought that a chokolisk was on the loose, but Torg (still not Potter) managed to survive the first wish without being turned to chocolate, used the second to turn everyone else back to normal, and fed the book to the weretoilet.
In 2005, Weaslo Ronsnap's familiar Sneezy was exposed as being William Wotcherclaws, an animagus who had secretly been You-Probably-Don't-Know-Who's servant all along. However, he later escaped custody and found his way to the weakened, baby-like Stoop. With a servant at his side, Stoop was able to begin planning his return. In 2008, traces of him began to reappear in historical records, and Gandledorf was able to remember his name.
Sensing Stoop's return, Gandledorf had Torg's bloodentity exchanged with that of Torg Potter and sent him a goblet with giblets in it in order to jibbort him to him. Unbeknownst to Gandledorf, his messenger was replaced by Wotcherclaws, who used the opportunity to get a sample of Torg's blood - the first step in Stoop's complicated plan to obtain Torg's blood. Gandledorf tried to warn Torg of the imminent threat posed by Stoop, but Torg got fed up with his inability to speak straight and ported back. However, he was dragged back to the school when the blood stolen from him was used to rig the Goblet of Flameyness to choose him as a competitor in the Try-Gizzard Tournament, forming a magically binding contract. When he touched the winning cup, the giblets placed inside it caused him to teleport straight into Stoop's presence. There, his blood was used as part of the ritual that enabled Stoop to be reborn in an adult form again. Stoop summoned his old minions to him and set out to create himself a new name by anagramming his old. Unfortunately, "I am Lord Moldypants" was the least stupid option he could come up with, so he settled for that. Meanwhile, Torg had escaped and found a shotgun, which he was not afraid to use. The wizards fought back with curses, but many were injured, and even Lord Moldypants was forced to retreat. Torg escaped for the time being.