The darkness a dark fog that covers the lands created by beasts ( boss or mini boss status) takes control of the dead similar to how you where revived but instead your body movements are deranged almost zombie like, there health significantly increased they cannot be put down by just headshots and dismemberment. If the guardian remains in it for too long without protection he dies and a darkness covered guardian spawns. ( if the traveller does turn out to be evil another version of a ghost will be within the beast and will fall when the beat is dead