Reward: 200 Renown, 1500 Gold.
The secret path to Bargate Prison, where your mother is being held captive, leads through Lychfield Graveyard. Head that way and talk to the gravekeeper. Take an ancient helmet from his house. It's a part of an armor of an ancient knight known as Nostro. You have to find all the parts of the outfit and take it to the crypt at the eastern end of the cemetery. Only this way will the Demon Door open before you. You have to search through the whole graveyard to get them: dig through all the graves (in places where your spade appear as one of the default items under F1-F3), open the crypts and fish in the river (at the bubble spot).
Nostro will open the path before you. The next area is quite a nice place to pump up some experience points (check the beginning of the guide). Eventually you'll get to the Circle of the Dead. Go into the green light in the middle. The Undead will appear. They materialize after entering the circle, so just wait for them to come closer and then attack. In the next 'round' you will face 3 stronger and bigger skeletons. Once you kill everyone, the Prison Path will be open.