How to get this quest
You get this quest automatically after completing The Hand Of the King.
Finding an expert
Once you have completed Fortress Of Wrath, talk to Mira in the Adventurer's guild.
Obtaining the letter of recommendation
The letter is obtained at the end of Mansion of Lust.
Obtaining the Humiltas Rune
Once you have the letter you can talk to the guard in front of a door north of Trademond. You will get access to Lord Trademond's mansion. You have two ways to obtain the rune:
Obtaining the Humilitas rune concludes the quest.