The Harmony is the idea of peace for the elves that Sylanna bestowed on them. The idea that everyone may be in harmony with nature and have unlimited understanding.
This idea is what all elves strive for, true enlightenment of the celestial heavens. They believe they they will end all wars and make peace and end all violence. But one thing stands in their way: the demons of Sheogh that will fulfill the opposing twisted ideals of Urgash and the Demon Overlords, or Kha Beleth, the Demon Sovereign.