I thought they been wiped out at the second war and all the War turtle fleed northrend and now serve the Kaluak as transportation.
but wait....... Dalaran too.....Mind Blown!
And BTW, according to the lore, Death Knights were Human paladin knights that been raised by ogre-magi(led by Cho'ghal) after acquisition of some sort of elvish runes from what we know as Scholomance hence the rune blade symbolysm of the WoW DKs.
I didn't find the right solution from the Internet.
http://www.mmo-champion.com/threads/118 ... Dark-Horde