You will receive a Night Jar and a Ritual Dagger. Using the Night Jar will allow you to place it on the ground. Clicking on the Night Jar after laying it on the ground will give you the option of using the Ritual Dagger.
The raven perched on the stone will start squawking and Aosoth the Crone will spawn (level 66^^^) to fight you. Kill her and bring her head back to Ihriel for your reward.
Note:If more than one person in your group needs the kill, only one person needs to place the Night Jar and use the Ritual Dagger, for all to get the kill credit.
See discussion page regarding problems with this quest!