Are Death Knights actually dead? It's a question that comes up far too often in places like LiveJournal's warcraftsues community, which has seen a parade of beautiful, warm, unrotted, pregnant not-a-Death-Knights in the past year. But putting aside people who want to have blue eyes and pretend to be a High Elf, people who want to be Drow, or people who are just in it for the ERP but for some inconceivable reason decided they needed to roll their super-sexy chick with blue-black skin, just how dead are our characters? And how did they get that way?
If you walk around Acherus during the opening quests, you'll see necromancers standing around meat wagons. The Scourge need to bolster the ranks of their new Death Knights, and quickly, so they've trucked in bodies to raise to do their bidding. Where do these bodies come from? It's not stated explicitly, but it's a safe assumption that they're soldiers who recently fell in battle against the Scourge. It's also a safe assumption that the bulk of them are from the Argent Dawn, since they have the largest presence in the Plaguelands. This is your "vanilla" backstory for the majority of the Knights of the Ebon Blade: A soldier or adventurer who fell in battle against the Scourge shortly before the events at Light's Hope Chapel and was raised in Acherus, then twisted by the Scourge there into a Death Knight. So the average Knight of the Ebon Blade was killed and raised into undeath. Who and what they were before is open to interpretation, but that much is pretty straightforward.
If you're an NPC, or if you just want to do things differently, it's also very possible that you were killed and raised at another point in the war. Darion Mograine, Highlord of the Knights of the Ebon Blade, died when he impaled himself on his father's sword Ashbringer. He freed his father's soul from the corrupted weapon but cursed himself to undeath, though it's unclear whether the sword raised him or Kel'Thuzad did. Thassarian died at the beginning of the Third War at the hands of Arthas' lieutenant Falric and was raised to join the Scourge as a Death Knight. Koltira Deathweaver was killed and raised by Thassarian during the Scourge invasion of Quel'thalas, then trained as a Death Knight. I admit my own Dariahn follows a little of both formulas, though it was more because I had his backstory hammered out a good few weeks before I played through the starting area than because I was striving to be different. He died in Stratholme and was raised to be a necromancer's personal servant, served the Scourge and the necromancer for years, but was not a Death Knight until he was sent to Ebon Hold by his master to train.
But can you become a Death Knight without dying? There are certainly examples of it in the lore, not least being Arthas himself. (Another would be a certain Drakkari you encounter in Northrend, but I'll not spoil that amazing quest-chain.) But the resulting Death Knight is no less undead. Just because they did not suffer a specific trauma that rendered their body dead does not mean they haven't undergone physiological changes that make them undead beings. Their tooltips even say they're Undead! A major factor in this conversion is the runeblade itself. When Arthas first encountered the dreadlord Tichondrius, he told the now-undead prince, "The runeblade that you carry was forged by the Lich King and empowered to steal souls. Yours was the first one it claimed." The RPG books have some useful hints here as well, as they list rules for Frostmourne as a player-wielded weapon: "An individual who wields Frostmourne will not part with it willingly. Over time the person will go from good to neutral and finally to evil. A non-undead evil wielder will then become undead. Finally the sword is able to suck the being's soul into the sword." This form of conversion likely has to be undertaken with some degree of consent, with the person willingly taking up the soul-stealing weapon. That doesn't mean brainwashing and other forms of manipulation can't be involved. Even Arthas didn't know what he was getting into when he took Frostmourne. But a Death Knight's runeweapon can cause those physiological changes in the living, turning them undead without the body ever technically dying. It's also worth noting that Scourge necromancers tend to show symptoms of undeath due to their constant exposure to necromantic energy, but they never seem to be classified as undead. Practicing necromancy alone doesn't seem to be enough for a spontaneous death-free conversion.
This brings us to the subject of metaphysics in Warcraft, how it applies to both Death Knights and Forsaken, and game mechanics vs creator intent (which is a subject I've spoken on before). Warcraft has the benefit of being one of those universes completely unlike our own where metaphysics are pretty much a known quantity. It's hard to say "There is no Light" when you keep getting hit upside the head with great hammers of it whenever you go into a contested area. Likewise, regardless of any of your personally-held beliefs about the real world, our characters have souls. While the Forsaken may be classified as humanoids so paladins don't destroy them in PvP, it's also a reasonable assumption that Sylvanas "freed" them by freeing their souls from the Lich King's grasp. That's why standard undead rules don't apply to them: They're a sort of hybrid creature with a soul in an undead body. It may have started as a bow to game mechanics, but it works.
How does this apply to Death Knights? The bulk of the Scourge may be mindless, and your average Forsaken was one of these, but there are numerous soldiers among them who are able to act and think on their own like the Death Knights trained in Ebon Hold. But they still serve the Lich King unquestioningly and are unable to act against his will, and this can be extrapolated to be because he controls their souls every bit as much as the mindless ones. Death Knights' souls are absorbed into their runeblades, which are Scourge creations. (Liches, as a side note, keep theirs in phylacteries.) But when you're sent back to your faction's capitol for what I like to call the Walk of Shame, you're given a letter from Tirion Fordring declaring that you have "the soul of a champion. A soul that has only recently been reunited with the body. " Whether it was because the power of the righteous souls at Light's Hope returned your soul or the Lich King freed the Death Knights as a way of casting them out after Mograine's betrayal is up for debate, but your soul is now your own.
So Death Knights are, like Forsaken, a sort of hybrid undead that once again has a soul. Some are raised from the dead, and others are enslaved and converted from Living to Scourge, but they are all free-willed undead. The Forsaken are usually just a little worse for wear.