Some of the Spirits, mostly in the western area of the forest, are neutral but many are also hostile to all players. They appear to be the spirits of the ancient kaldorei race. Exactly why the spirits are here is unknown. It's possible they were killed by the dragon magic that transformed much of the forest into its current crystalline state.
The spirits are also act more like city NPC's than other usual killable NPC's would in areas, as some spirits sit and listen to others talking inside the ruins, while others can be seen sitting quietly, and others still can be seen jumping up and over furniture in the ruins, almost like they are excercising. This leads to speculation, that perhaps the spirits have either come to accept their undead condition and have learned to exist with it, or perhaps they might not even be aware they are spirits at all and contiunue to exist like nothing had happened, and they were still alive. This kind of undead is one of the rare seen "fearable" undeads.