Stalvan: The Early Years
The first thing you really learn about Stalvan in the Legend of Stalvan quest line is that he apparently hailed from Silverpine Forest. Nothing is really known about his status or life there. It seems possible he was part of some noble family, since there is a Mistmantle Family Ring. He also lives in Manor Mistmantle, which could conceivably be an old family holding, although he may also have simply redubbed it that when he moved there. Regardless, any of Stalvan's family that stayed behind in the area seem to have either succumbed to the Scourge or changed their name out of shame for his later deeds.
At some point made his way to Stormwind to study under a Headmaster Crillian at an unnamed Academy. It seems likely he left somewhere around 7-10 years ago, since he was in Southshore during Thrall's prison break according to the Caverns of Time. If he was heading straight to Stormwind, he would have been there soon after.
What he was studying is unclear. Magic might be a possibility, but Dalaran would have been closer and more prestigious, and Stalvan doesn't seem to have been slow witted or have had any other problems that would have caused him to be rejected from Dalaran though. It's possible, of course, that his family had connections in Stormwind, or he was studying some general course of academics that Crillian was renowned for teaching. Whatever it was, after a few years, his Master urged him to go out into the world and increase his knowledge and wisdom.
After leaving Stormwind, he came to Moonbrook, apparently just in time to become the first teacher of the new village schoolhouse. He was, by all indications, very well liked there. The next time he moved was to Goldshire, where an undelivered letter reveals that he was looking for a new job after leaving Moonbrook because of the state of events in the region (presumably the Defias moving in), specifically as a tutor to noble children. Strangely enough, that letter never got to its intended recipient, Lord Flintridge, but he was hired anyway.
This short stay in Goldshire may be the first sign that not all was right with this seemingly charming man. Apparently, the courier charged with delivering his letter was spooked in the night by some unknown thing, and ran out of the Inn in his nightclothes, never to return. Since this would have, in theory, happened before Stalvan was ever hired as a tutor, and before he committed his heinous act, It suggest he may have been up to shady things. Maybe the reason he left Moonbrook was related to this instead of being trouble with Defias.
Regardless of the undelivered status of the letter he sent to Lord Flintridge, he was eventually hired as a tutor to Lord Flintridge's Children, Giles and Tilloa. However, in the process of learning that, we also get some idea of what really happened, as we head to the Flintridge Manor in Stormwind only to learn that their Caretaker is packing up the last of the Flintridge estate to be auctioned off to pay back taxes. Apparently, the whole family was massacred.
From a torn journal page among the Flintridge estate's leavings, we learn that Stalvan found Tilloa to be a comely young maid "on the cusp of womanhood," and that they were headed to the family's summer home at the Eastvale Logging Camp in Elwynn Forest.
In Eastvale, you find yet another piece of the puzzle in another journal page. Stalvan apparently began to fall in love with Tilloa, and felt she reciprocated the feeling because she gave him a flower once. Yeah, essentially, he's a modern day Illidan, except even creepier.
The trail would end there, except that the new inhabitant of the old Flintridge summer cottage remembered hearing some stories about Stalvan when he hung out at the Scarlet Raven tavern in Duskwood. Luckily, it seems the Innkeep there keeps tabs on local lore and found some muddied journal pages. The pages finally confirm Stalvan's ultimate descent into madness. Tilloa brings home a fiance, and calls Stalvan a "nice old man" when introducing him. Apparently, this was suddenly enough to push him over the edge. In a rage, he accused Tilloa of betraying and mocking him.
What's likely is that Tilloa saw him as a favored tutor and a good mentor, but never considered the idea of being in love with him, since he was an authority figure and an older man. It's unlikely that she ever hated him or meant to scorn him, but unfortunately, hell hath no fury like a Mistmantle scorned.
In the last page of the journal, he reveals his plan to kill Tilloa and her family. He followed through on it too, killing Tilloa, her fiance, her brother, and her parents in the massacre alluded to by Caretaker Folsom.
From there, your task is straightforward. You go to the authorities, who confirm Stalvan's identity and ask you to kill him. You go to his "manor," a ramshackle hut with a yard full of ghouls and an herb called the Tears of Tilloa, to find that he has somehow turned undead and is surrounded by Ghouls who may serve him, or at the least leave him alone. He attacks you, you kill him, and bring his ring to Madame Eva. Then you get your choice of two rewards: One is an axe that he probably used to murder the Flintridge family, the other his ring.
And thus ends the Legend of Stalvan Mistmantle.
The Man, The Myth, the Legend.
How Stalvan deserves the dark powers and the legend he's gained are honestly unclear to me. Make no mistake, what he did was rather heinous. He killed at least 5 people, possibly more if he killed the Flintridges' other servants or any bystanders. But was the crime itself so bloody and gruesome that it attracted the necromantic energies and damned spirits that flock to Stalvan's old haunts and protect his past?
There's two theories I can think of as to why this is so. The first is that he lives in Duskwood. Duskwood is a place that is currently in the grip of a powerful darkness. It seems a magnet to dark forces and misfortune. It may be that the darkness gripping Duskwood or the demonic power radiating from nearby Karazhan somehow latched on to Stalvan's insanity and negative emotions and manifested all these powers and turned him undead.
The other possibility is that he's always been a Necromancer. Perhaps the mysterious headmaster Crillian was a Necromancer or a Warlock. This would also match up with the mysterious happenings with the undelivered letter in Goldshire. Certainly if he's had Necromantic powers all along, it would explain why he's summoned ghosts to help protect his past and ghouls to help protect his manor. It might also explain why he's kidnapping little girls: Perhaps he's hoping to experiment on them to find a way to raise Tilloa. That would also explain why he's now undead: He may have turned himself into a Lich.
There's also the question of why a "legend" has built up around him so quickly. His infamous massacre could not have happened long ago. By all indications, he left Silverpine less than a decade ago, he left Moonbrook only a few years ago, and the Flintridge estate has only just been completely liquidated. Could a legend really pop up so quickly? Yes, he is a Mass Murderer, but most of us kill more people every day when questing, some of them not much more able to defend themselves than Stalvan's victims.
Perhaps it's the jilted lover theme, or the fact that the Flintridges were a noble family, thus adding a "celebrity" bent to the whole deal, and a bit of glamour. In addition, the idea of an insane criminal living on a haunted hill and dragging people to their death is similar to many an old ghost story, so you can also consider Stalvan to have essentially stepped into a preexisting type of legend.
Then again, Caretaker Folsom mentions that "his father" mopped up the blood from the Massacre, which would suggest it happened some time ago, when his father was the caretaker of the Flintridge estate -- a suggestion that doesn't jive with a timeline that would involve Stalvan leaving Silverpine Forest to come to Stormwind only a decade ago at most. Of course, this and the fact that Stalvan's original letter to Flintridge went undelivered but he was hired by the Flintridge family anyway may simply be examples of plot holes that were never noticed and plugged before the quest line was implemented.
The source of his unholy power is unsure, and his legend is perhaps bigger than it deserves to be, but all the same, Stalvan is now a fixture of WoW lore, the story of a creepy jilted lover who took things way, way too far. Maybe one day we'll get the full story as to just where he came from, what he learned, and how he got all those crazy necromantic powers. But for now, he's at least a decent amount of experience of a leveling alliance newbie, a good excuse to explore the Human lands, and another piece of Warcraft's new lore.
Wrath Spoilers Ahead!
One more little tidbit from Stalvan's past: It's interesting to note that a massacre wasn't his first choice to take his revenge. Before he decided to kill Tilloa and her family, Stalvan actually went to Dalaran at some point and tried to coerce a potion or charm from the Mages to make her fall in love with him. The Mages either couldn't or wouldn't do a thing, so Stalvan departed, leaving behind a bitter message for them -- one you can fish up in Dalaran come Wrath of the Lich King.