Originally Posted by Blizzard Entertainment
It would be reasonable that she either couldn't raise them (can you raise what was risen by the LK already?), or could but they would no longer be death knights (but mindless ghouls/skeletons/whatever). Unless I'm missing something.
It really isn't documented but I assume it's a case of similarities. Vry'kul are essentially humans that have been less altered by the curse of flesh. Dark Val'kyr are female vry'kul that have been given power by the Lich King. Their original purpose was to judge other vry'kul in the Lich King's service and as punishment they would resurrect them as malformed and misshapen undead.
It wasn't stated, true, but that's what I assumed. We know that Sylvanas could raise dark rangers, thanks to https://wow.gamepedia.com/Dark_Ranger_Velonara
Dark rangers or not, the high elves were being raised during vanilla.
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Death Knights can be risen again. While most of the examples we have in Wrath are (allegedly) because of Valkyr, it's unclear if Sylvanas could utilize Valkyr to the same extent before - now, who knows what she learned about Death Knights after messing with Koltira. Freshly risen Forsaken currently have free will though, so even if Sylvanas did raise a fallen Night Elf Death Knight the Night Elf would have to choose whether their loyalty to their people is more than the favor of granting them life again. Maybe if Sylvanas could counter-act some of the negatives that being a Death Knight has (the constant pain and hunger) then maybe they would even feel indebted to Sylvanas. This could especially work if the Night Elf Death Knight was shunned by their people or didn't really get along with anyone. If the Night Elf didn't really have a place with the rest of their kin, and Sylvanas just cured most of the negative Death Knight pains and hungers then I could definitely see a rogue NE DK working for her.