Wiki - The Sunless Citadel Entrance Quest
Part One - Unwanted Demon
Those who would embark on the Journey to gain entrance to the Sunless Citadel must complete a chain of Quests that begins just outside the Moonglow Castle of Magicka, in Felucca, with a Woman In Distress.
Moonglow Castle of Magicka
Part Two - Evil Potionry
Travel to Umbra in Malas and Seek out The Potion Seller. Here adventurers learn that the vendor mistakenly sold the Woman in Distress a potion of darkness instead of what she asked for.
The Potion Seller charges the hero to destroy the loosed demon, An Aberration, for which a real Potion of Drunkenness will be given.
Seek out the Aberration back on Moonglow Island where The Woman in Distress released it. The demon must be destroyed three times and each time will reappear closer to the graveyard north of the Castle of Magicka. It's locations are marked 1, 2 & 3 on the map to the right. When the demon is killed the third time then return to the Potion Seller to collect the reward.
Toggle the Potion of Drunkenness as a Quest Item and return to the Woman in Distress to complete The Unwanted Demon Quest (Part 1). She will notice Bach's Journal, which was received for slaying the Aberration and suggests taking it to Tomislav who resides within Moonglow Castle of Magicka.
Part Three - Last Wish
Tomislav can be found on the third floor of the Moonglow Castle of Magicka. There he mourns his father and can't even bring himself to read the will. Instead he asks the hero to do it for him.
Within the letter, Tomislavs' father gives the first clue as to how to find his inheritance. Seems he's left a note somewhere in a tavern?
The first clue, as mentioned in the father's note, is in the tavern in Umbra.
The second clue can be found in Hanse's Hostel in Mook Town.
The city of rock refers to the Mook Town, which is well known for it's mining, and Luna is the city of light. Tomislav's Treasure can be found near a yew tree just about halfway between the two along the road. Steal the treasure (100 Stealing Skill Required) and return it to Tomislav and he will divulge that the ruins around where the treasure was found is, indeed, the Sunless Citadel.