It would appear from the Scourge ground activity that any fallen champion or hero of the Alliance or Horde are killed and resurrected as champions of the Scourge instead. They are led or controlled by high ranking necromancers and Val'kyr. The fallen heroes aren't the only dangers here though, there are also countless Frost Wyrms, Flesh Giants, and risen skeletons armed to the teeth. The only way to actually get through the valley is to fly. Walking will most likely get you killed by the dozens of mobs grouped together, most of which are elite.