So in the last few days, I've been having some interesting discussions about how the Death Knight class is going to work, lore-wise.
The Death Knights were created by Gul'dan, who slaughtered his acolytes and put their spirits into the bodies of fallen Stormwind defenders. So essentially, they were orc spirits in human bodies. Does this mean that only humans can be Death Knights? I doubt it.
I thought perhaps there would be a way to split the spirits from the bodies again. Or perhaps somebody has figured out how to train themself as a Death Knight without their spirit leaving their body, and can train others to do it. With the paladin/Death Knight in the WotLK trailer, this one seems the most likely.
And will our old friend Teron Gorefiend be involved in the plot? Given where he is now (Black Temple) I somehow doubt it, as it was stated that the questline would be one that anyone could do.
Teron Gorefiend. Only a mother could love him.
ahem *steps to the podium* While Teron Gorfiend and his ilk were the first Death Knights, created by Gul'dan in the service of his new master, Ogrim Doomhammer, and to replace his Shadow Council who were slaughtered by the aforementioned then Warchief, these were not the last. As anyone who place Warcraft 3 will tell you, we will soon face what is far more likely the type of 'Death Knight' we will become, Arthas.
The paladin, corrupted by the Lich King's machinations, was driven to do anything to attain the vengeance he sought, and saw the runeblade Frostmourne as his best tool in that, not knowing it would steal his soul, make him kill his own father for a crown that he was already destined to inherit, lead him into service to the Lich King, and eventually take up the mantle of Lich King himself, fusing his soul with that of Ner'zhul.
These are what you will find we will become, heroes driven to desperation, and falling into corruption. Course, might just be me. :)