Put simply, the Knights of the Ebon Blade are traitors to the Scourge. Like the Forsaken, they've broken free of the grasp of the Lich King due to the intercession of an external source - Sylvanas and the Forsaken escaped the Lich King's grasp when he was weakened due to the crack in the Frozen Throne and had to recall Arthas to Northrend, while the Knights were more autonomous than the average Scourge and regained their freedom following the events at Light's Hope Chapel. At LHC, the Knights were freed due to the actions of their Highlord, Darion Mograine.
If you played through the events of the Death Knight starting area, you already have a basic idea of the origins of the order, especially if you ran around Acherus reading books like I did. Some are funny, some are meaningful, and from them we can glean certain information about the history of the order before the events leading up to its break from the Lich King. If you've read the Ashbringer comic series, you're of course informed of some of this anyway. But there's still devils in those details for us to discuss.
Basically, Alexandros Mograine found a crystal object at the battle of Blackrock Spire in the hands of an orc warlock. We know from Christie Golden's Rise of the Horde that it was most likely one of the fragments of the Ata'mal Crystal captured by the orcs when they slaughtered the draenei. (Possibly Leafshadow, the crystal used to hide the Horde's attack on Shattrath until it was too late for the draenei to defend themselves, although this isn't confirmed.) Whatever the crystal was, in the warlock's hands it was so suffused with evil fel energy that touching it destroyed Mograine's hand. Years later, casting holy magics in an attempt to destroy the crystal instead purified it, making it a beacon of the Light itself. So inspired, Mograine left for Ironforge, where Magni Bronzebeard himself forged a sword out of the crystal for Alexandros: both the sword and its wielder would become known as Ashbringer.
Sadly, it was this man's legend that would cause the ruin of both his sons and the indirect formation of the Knights of the Ebon Blade.
Alexandros Mograine had two sons, Renault and Darion. Both were fairly young when the Scourge first invaded Lordaeron... we see them as old enough to get into trouble going into their father's chest (where he kept the crystal from Blackrock Spire) but young enough to get scolded for it. However, both are close enough to adulthood to be taken into combat at Stratholme some years later on the fateful night when Balnazzar usurped the body of Grand Crusader Saiden Dathrohan. Renault grew up resentful of the attention Alexandros gave to Darion, who reminded him of his deceased wife Elena. (It's also possible Renault resented Darion because his mother died giving birth to the younger boy.) Renault was more like his father, quick tempered and as we all know, the more fathers and sons are akin to one another the more chances they have to butt heads.
Renault's seduction by Balnazzar from within the Grand Crusader's body led to Alexandros' death and resurrection as one of the Four Horsemen of Naxxramas, one of the Lich King's most powerful Death Knights. The potent Ashbringer was thereby corrupted. Note that while the Death Knights of Naxxramas are indeed servants of the Lich King, they are not members of the Knights of the Ebon Blade.
In the Ashbringer series, it's revealed that Darion Mograine doesn't leave things to this state of affairs, but rather joins the Argent Dawn and manages a daring raid on Naxxramas itself. (This raid also is key to Varimathras and Putress' later actions at the Wrathgate, as it provides them the basis for the plague they use against the Lich King, Alliance and Horde.) After escaping from the Four Horsemen with the Corrupted Ashbringer (at the cost of his entire party, which I have to say is kind of cold... basically, Darion ninja's the sword and drops group, not cool, didn't even wait for them to res) he did what anyone who was totally unaware that it was his brother who killed their father in the first place might do and went to his big brother for help.
Yes, that turned out about as well as you might expect. Renault attacked Darion, Alexandros' ghost comes out of the sword and kills Renault, Darion is pretty messed up in the head over the whole thing. A visit to Tirion Fordring out in his shack in the Plaguelands (where Tirion, still in his depressive exile phase, basically was no help at all) ended with Tirion telling Darion that only an act of love greater than the act of evil that bound Alexandros to the sword could free him.
The first battle of Light's Hope Chapel followed hard upon, which is difficult to summarize without sounding somewhat irreverent but I'm on a schedule here. Darion returns to Light's Hope, Maxwell Tyrosus gives him a guided tour of the crypts beneath the chapel containing the thousand protectors who died fighting Arthas at Lordaeron and who were moved to LHC to prevent his perverting them into undead minions. If there's one thing Arthas loves to do, it's perverting someone into an undead. He can't get enough of it. Even when it repeatedly comes back to bite him on the ass, if there's an even half-way decent dead person around Arthas'll raise that sucker in two shakes of an abomination's back arm. So the Scourge attack, the Argent Dawn defend, Darion whips out the Corrupted Ashbringer and bucks wild until he finally reached Kel'Thuzad who snarks at him that not even the Ashbringer can destroy him and Darion decides the ultimate act of love that would redeem his father's soul is to stab himself in the chest with the Ashbringer.
What amazes me is not that it works... I'm sure there's some form of logic there... but that Darion's sacrifice causes those thousand champions of the Light to bust out and destroy pretty much the entire Scourge army except for Kel'Thuzad (guys, please, while you're wiping out the Scourge? Kill the floating skeleton in the headdress and chains too, okay?) and our friend Darion, who is now an undead Death Knight in the place of his father. The reason this amazes me is that, once he's the head of the Lich King's brand new order of Death Knights, Darion leads them into battle at Light's Hope Chapel to be defeated by the very same power of the Light he himself unleased in the previous battle. Apparently undead makes a man forgetful.
From there, Darion heads up the push to create a new order of Death Knights for the Lich King. This leads to the events of the Death Knight starting zone, where your DK plays a pivotal role in wiping out the remnants of the Scarlet Crusade who hadn't managed to get on boats and sail to Northrend, and from there to the second assault on Light's Hope Chapel where the power of the Light (and possibly Alexandros Mograine's redeemed spirit) combined with Arthas' cartoonish super villainy (I mean, seriously, showing up and admitting that you fully intended all of your new Death Knights to die as long as it got you a shot at Tirion? It's one thing to do it, sure, we've all set up complicated long distance schemes to lure our archenemies out where we can attack them, but to just come right out and admit it? Lame, dude) helped break the Lich King's hold on Darion. From there, the Highlord gave up his hold on the Corrupted Ashbringer, allowed Tirion to redeem it, and turned all of the strength of will (some might say 'ridiculously fanatical single minded drive') that helped him beard Naxxramas and rescue his father's spirit to the task of destroying the Lich King.
Throughout Northrend, in fact, it's been members of the Knights of the Ebon Blade who have scored some of the more crucial victories against the Lich King. While the Argent Crusade tried to hold Zul'Drak, it was the Knights of the Ebon Blade who struck down Drakuru. It was the Knights of the Ebon Blade (led directly by Darion) who helped stem the tide of disaster and preserved the first breach into Icecrown itself. And when Tirion Fordring nearly gets himself killed by the Lich King, it's again Darion Mograine and his death knights who have to pull him out.
In patch 3.3 we'll see even more of this with the Ashen Verdict, as following the somewhat dubious events of the Crusader's Coliseum the Argent Crusade and Knights of the Ebon Blade have joined forces to take the fight to Arthas once and for all. Darion's up front inside the Citadel leading in a snazzy set of T10 DK armor. As to the future of our last surviving Mograine? Well, there's some lore spoilers in the new five man instances (and if you've read this far into this post you're probably not terribly concerned about being spoiled but I figured I'd warn you anyway) that indicate that post Arthas, someone else has to step up and take over the Scourge, or it will buck wild and eat everyone. (Why Arthas hasn't already ordered it to do exactly that is probably best explained by his neurotic need to convince himself he's right and that everyone would break just like he did - half of Arthas' appearances in Northrend are basically to taunt you, something he seems to have picked up from Mal'Ganis) Darion's probably not the lead contender for this job, but I'd put him solidly as a dark horse for it. He's also heavily involved in the Shadowmourne questline. So we're certain to see a lot of him in patch 3.3 at least.
Patch 3.3 is the last major patch of Wrath of the Lich King. With the new Icecrown Citadel 5-man dungeons and 10/25-man raid arriving soon, patch 3.3 will deal the final blow to Arthas. WoW.com's Guide to Patch 3.3 will keep you updated with all the latest patch news.