4. The Ritual
After you gained enough renown, go back to the Temple of Light and speak with the Abbot. He'll send you to the cave, to accompany a monk named Hannah. At the first spring there is a silver key sitting near the pressure plate you have to stand on. As you exit the first spring there will be two groups of Hollow Men that you'll have to kill, these are a little stronger than the Hobbes, but still isn't too bad. In the second spring you have to protect Hannah as she is filling the jug, the fastest way is to shoot them from the plate and use your will abilities against them, this way you can protect her while standing on the pressure plate.
The third room you'll fight a few waves of Hollow Men, follow by a Headless Hollow Man (boss). He has hard melee and the Shock spell that can wreck you if you aren't careful. The easiest way to kill this guy is to stay at a range and shoot him with spells, if he gets close get a couple hits in and get your distance back, eventually he will go down. In the next room you'll be informed of something and complete the quest after you go back to the Temple of Light.