I'm not sure where I want to start with the story of The Knights of the Ebon Blade, a faction that some feel is truly the heart of the Wrath of the Lich King expansion. Not since Vanilla and the Cenarion Circle have we seen a group in which the CLASS you are can effect your reputation gaining abilities. There is no way you can discuss these guys without looking at Death Knights in general so lets begin there and move forward until we get ourselves to present day.
The Death Knights for Blizzard have undergone several reworkings. When they were first introduced in Warcraft II: Tides of Darkness they were really little more than mounted Liches. At that point in time Death Knights were the souls of fallen orc warlocks residing in the claimed bodies of Alliance Knights. With Warcraft III: Reign of Chaos they decided to get rid of the middle man and just go about corrupting former Paladins turning them into puppets of the Lich King. Unlike the original Death Knights this new incarnation were apt at both close quarter combat as well as ranged increasing their ability for terror on the field.
This brings us to their PRESENT incarnation. Now the ranks of the Death Knights are filled with both the fallen as well as the willing and deceived from various disciplines. Gifted fighters, their present abilities reflecting the previous two incarnations with a new twist. Their ability to control their own actions have been greatly increased, to the point that some of them have managed to escape the grasp of the Lich King and now work independently.
After the Battle of Light's Hope Darion Mograine gathered those willing to defect with him and took the floating necropolis Acherus over the Scarlet Enclave in the Easter Plaguelands as their new home. Once that base was established as a neutral ground for ALL Death Knights he sent representatives back to their original factions with letters from Tirion. With the former heroes once again welcomed amongst the Alliance and the Horde the Death Knights of the newly formed Ebon Blade fixed their eyes on Icecrown and gaining retribution for all that they have endured.
If you play a Death Knight you will reach friendly with the faction shortly after you gain your freedom from the Lich King, all other classes begin at neutral and really won't encounter the Knights of the Ebon Blade until Northrend. Most players will first begin gaining reputation with this faction at their quest hub in Zul'Drak. Here the Knights of the Ebon Blade are working in conjunction with the Argent Crusade but it is still VERY early in their relationship so distrust and disdain between the two factions is evident.
By the time you get to Icecrown and encounter the Knights of the Ebon Blade again Darion Mograin and Tirion Fordring are BOTH present so the everyday troopers are on their best behavior. However if you manage to take the time to listen to Darion (disguised as the Ebon Watcher) and Tirion discussing tactics at the Argent Vanguard you will quickly realize that the two have VERY different ideas about what it will take to win against Arthas. This divide is further demonstrated at the Argent Tournament where Death Knights have their own SEPARATE quest givers from those of other players. (Though the quests given are exactly the same.)
Aboard the flying ships the Skybreaker and Orgrim's Hammer players will encounter representatives of the Ebon Blade working with the Alliance and Horde military. Again the basic troopers express their discomfort with working with the Death Knights but by this time the specific representatives have earned their grudging respect. Thassarian and his friend Koltira Deathweaver start players on the quest chain to create a Ebon Blade stronghold, this time in the recently purged Shadow Vault. Once players complete the "Its all fun and games" quest line you gain access to both the Ebon Blade quartermaster and a wide range quests which will quickly get you to exalted reputation.
In total the Knights of the Ebon Blade offer 112 quests. Of those only seven are Death Knight specific. Eleven are daily quests all located in Icecrown. What this means is that while you won't really be able to work on gaining reputation with this faction until about level 77, it is VERY easy to get after that point. Once you have access to the quartermaster in the Shadow Vault you can also obtain a tabard and gain reputation while running dungeons as well.
Why is gaining reputation with these guys even important? Because at Revered you can purchase the helm enchant Arcanum of Torment which for the vast majority of dps classes is the enchant of choice. When Wrath was first released they also sold some of the best introductory epics available, however with the introduction of the tier gear for badges system this isn't as much the case any longer.