If I have to make a call here I would say they want to harvest the mane of all spirits for something.
Unlike what some may think they are not trying to "save" the spirits but harnest their power truning them into that black stone.
Also in that state the spirit is not dead but asleep. I have no idea how to awake them but if anything I would imagine giving it mana would do the trick since they fall like that when they are without mana.