Yag's power is good against certain ncps in Mistlands that you will have to interact with in some fashion for progression. Beyond that like other's stated it is just the wisp which makes stumbling around usually blindly in the Mistlands just stumbling around.

Killing Yagluth will enable Mistland events until the Mistland boss is killed.

edit: It's possible to spot soldiers stomping around in the mist and if you are attacked by 3 seekers (usually 1 is starred) that will indicate that an Infested Mine may be nearby. Just don't run around at night.