The Horde's primary Death Knight representative, Koltira Deathweaver, has gone through some interesting storyline developments in Cataclysm. When we first meet Koltira in the post-Shattering world, he's working directly for Sylvanas, leading the Forsaken effort to defeat the Alliance forces at Andorhol.
When I started doing these quests, I thought it slightly unusual since Koltira and Sylvanas had never worked together or shown any kinship in the past. She refers to herself as Koltira's queen, something she doesn't do with any other non-Forsaken. And he in turn treats her with a great deal of respect, calling her "highness" and addressing her with a reverence that you normally only see coming from actual Forsaken.
However, after giving it some thought I realized it mostly made sense. The Plaguelands are an area of disease and corruption that are being cleansed and redeemed, a suitable analogy to the Death Knights themselves. And where else would Koltira feel at home now, except on a battlefield? This was probably the place where Koltira felt his talents/experience could best benefit the Horde, amongst allies who wouldn't distrust or be scared of his ex-Scourge status.
However, another thought occurred to me. Compared to original (Vanilla) Forsaken such as Sylvanas, Koltira is generally regarded as a "newer" member of the Scourge, since we didn't meet him until recently. But he's actually been dead a long time. He died defending Quel'Thalas/Silvermoon against the Scourge, mere minutes after Sylvanas was slain. He's actually been undead just as long as she has.
Actually, considering that Sylvanas was a Banshee slave for some time, while Koltira was raised not as a slave but as a valued Death Knight with independence and (relative) freedom, you could almost say that he's got more experience than her in their new existences.
I generally consider Sylvanas as a Forsaken and nothing else, but these two ex-High Elves - the only ones anywhere near Andorhol, incidentally - made me ponder any similarities.
Examining Koltira's death made me realize something else - in life, Sylvanas and Koltira most likely knew each other. As Ranger-General of Silvermoon, Sylvanas was in charge of the High Elf military and the defenses of their lands. Koltira was one of the guardians of An'owyn, a temple of vital importance to Quel'Thalas' security, so he definitely would have been under her command.
How close were they? Unfortunately, this is impossible to say, at least from the information currently available. All we know is that Koltira didn't appear to be high in the military hierarchy - he wasn't in charge at An'owyn, judging from the identical armor and trappings he and his late brother wore, and when he later led Silvermoon's defense against Arthas' forces, he didn't issue any commands or orders to the other elves. (Though he was pretty awesomely defiant.)
Since we don't know Koltira's military rank, it's pretty tough to speculate how well Sylvanas and Koltira may have known each other. Koltira would obviously have been very familiar with his amazing Ranger-General, but perhaps Sylvanas didn't even know Koltira except as one of Silvermoon's many soldiers.
(This is where the slash writers deviate into pages and pages of Sylvanas/Koltira fantasies, I imagine.)
Still, I think this might be one of the reasons Koltira decided to join up with the Forsaken forces - to attain some semblance of his former existance, working once again for his former general. After all, she was amazing in life, she's supposedly even more impressive in death, and best of all, she's someone who knows his pain and could relate to/understand his inner turmoil.
Of course, that makes Andorhol's events even more alarming and chilling. Forcibly brainwashing an ally is bad enough, but when that ally is a former military comrade, a brother-in-arms, and possibly even an old friend...well, you know what Garrosh would say about karma.