At the battle for Mount Hyjal, Malfurion Stormrage sacrificed the power sustaining the night elves' immortality to defeat Archimonde. Surely some disagreed? Today there are reports that someone or something is draining the power from the land around Hyjal Summit: snowy peaks, a river cutting through ancient elven ruins and the whispering of ghosts from the Third War.
Blizzard hasn't published the bosses yet. We'll add them as soon as they're known (the beta starts on September 17).