You receive this quest from the Cauldren Witch in the Lake of Llyn Cau during the Cure Duchess Cyndianna quest.
The Cauldren Witch says she'll give you the Ritual Magic ability if you free her sister from the gasp of the evil creatures in the Caves of Llyn Cau beneath the Lake of Llyn Cau.
You will receive the Free the Witch quest.
Go to the Caves of Llyn Cau, which are located in the Southeast corner of the Lake of Llyn Cau.
Follow the path.
When you hit a dead end, go 2 Steps East, 2 Steps South, and 2 Steps East.
Open the gate and talk to the Ghost of the Lake.
You will receive the Jade Eye Amulet, which is for the Cure Duchess Cyndianna quest.
Hit the Northern Wall.
Go through and keep hitting the walls.
You'll come across a dark red wall in the Southeast corner.
Touch the dark red wall and it was disappear.
Pick up the key.
Use the key to unlock the Northeast door.
Kill the last Living Bog Man and talk to the Lake Witch.
You will receive the Lake Witch's Helm.
Return to the Cauldron Witch.
The Free the Witch quest will be solved.
You will receive an increase in fame.