Some folks noted last week that when I said Diablo II's necromancers were the granddaddies of WoW death knights, I might be discrediting the other major inspiration: Everquest's shadow knight. It's definitely true, the shadow knight looks at lot like the death knight, but the biggest reason I didn't discuss them last week is that I haven't played mine in probably close to 10 years. As soon as WoW was around (and I fixed my broken computer so I could play it), I never looked back at EQ. Still, I have a lot of fond memories of the game. Before WoW, Everquest was one of the most popular MMOs, and if you said "MMORPG" in mixed company, most anyone who knew what you were talking about would likely think of Everquest (except for you weirdos who preferred Ultima Online).
As someone who played my level 65 Erudite shadow knight extensively (back when level 65 was the level cap), I can instantly see why I gravitated to the death knight. Like death knights, shadow knights are considered a hybrid of warriors and necromancers. In EQ, necromancers were a playable class though, and many of spells shadow knights got were simply weaker versions of necromancer spells, or just the same spells capped at a lower rank/power level. Still, game play ended up somewhat similar to the modern death knight. You summoned your permanent pet (with a bone chip reagent. similar to early Wrath's Corpse Dust, but usually not vendor purchasable unless you were lucky), you loaded your enemy with diseases, and went into melee, often using health draining attacks to keep yourself alive. Much like death knights, shadow knights were often maligned for having easier soloing abilities than most other classes. Back in my day, that was mostly due to a combination of feign death and fear kiting, the former of which only hunters can do, the latter of which only some warlocks and shadow priests really use in WoW. In the modern era, shadow knights can often successfully tank solo much under the same principles as the death knight.
From a lore perspective, shadow knights are slightly different from death knights in that they are almost all evil to the core, drawing their powers directly from gods of disease, hate, and fear. Still, there's some nuance and fun to be had in playing evil. Me, I played an erudite for the backstory of the spurned exile, and because Paineel is the best city in all MMOs ever. Regardless, both Everquest and Everquest II are currently free-to-play, so if you want to go back and see how death knights looked in WoW's predecessor (or in the case of EQ2, one of WoW's dogged contemporaries who never quite got as popular) while you wait for the WoD beta, you should have ample opportunity to do so.
Lord of the Rings
While Sauron is the chief baddie of the Lord of the Rings books, we see very little of him directly. Most of menace comes in the use of the Nazgul, an elite force of 9 wraiths who track down his enemies and lead his armies. In a lot of ways, they definitely herald death knights. They are elite knights, humans who betrayed their former vows to join a dark lord due to despair, anger, and greed. They ride on unholy steeds, and their attacks drain the vitality of their victims, so that even if they survive the initial assault, they may still die of exposure. In a weird "chicken and egg" scenario, the recent Peter Jackson movies have a visual interpretation of Nazgul and Sauron himself wearing helms that suspiciously mirror the one worn by the Lich King.
It's not as easy to find inspiration for your death knight in the Nazgul, admittedly. They really are irrevocably evil, and beyond that, have no individual personalities, having been fully subsumed to the will of Sauron by the rings that give them their powers. Still, there's something in their steadfast implacability that you can admire and emulate if you're looking to role play one of those cold, emotionless, dedicated types. The Nazgul are, in some ways, one of the purest distillations of the dedicated and loyal knight, even if it's for the completely irredeemably evil side.
Dungeons and Dragons
Any discussion of MMO inspirations probably isn't complete without a discussion of pen and paper games, including the elder statesman of them all, Dungeons and Dragons. Many an MMO playing nerd, including many of the major devs and corporate officers at Blizzard, grew up with a d20 in their hands and a character sheet in their 3 ring binder. DnD inspired many of the basic mechanics and class sensibilities in the most popular mainstream MMOs.
Death knights aren't really one of the most core well-known classes in MMOs. For my part, I've stuck to Bards and Monks when I've played DnD, but have gotten most of my experience in home-brew games where I've played custom classes. Still, if you want to go with the classic DnD, Blackguard is probably your closest link to a death knight. It plays a lot more like a fallen paladin, with many of the same skills or corrupted versions of said paladin spells and skills, but that lines up rather nicely with the original "fallen paladin" death knights we got in the WCIII era. In lore and general class feel though, it can probably be considered an inspiration for the death knight, or at least something in the same spirit.
In other areas of DnD, death knights (by that name) often appear as NPC enemies. One of the most famous examples is Lord Soth, from the Dragonlance campaign setting and books. I find him to be kind of a creep, but some folks like him.
While it doesn't appear in every Final Fantasy game, the dark knight has been a quintessential Final Fantasy class (often called jobs in Final Fantasy games). The most iconic dark knight is arguably Cecil Harvey from Final Fantasy IV (sometimes called Final Fantasy II in America). You can also play the Dark Knight class in the Final Fantasy XI MMO. Final Fantasy IV was my first real JRPG, so I have fond memories of playing the dark knight Cecil, who struggled to keep his humanity while being forced to serve a dark power, wondering if his dark powers were worth using. Cecil's struggle and angst can be solid role play grist for a death knight taking the angle of the reluctant death knight who hates their powers and wishes they could shed them and be their former self again. The game was re-released in 2008 for the DS with updated graphics and new systems, so it's even got a relatively fresh version to try out if you have the hardware.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.