An Innocent Man | Act II - side quests Dragon's Dogma Guide

In short: Tomlin of Cassardis is looking for his father. It turns out that the men is rotting in Gran Soren's prison. You need to get him out by using two Skeleton Keys. Before that, though, you'll need to acquire some information from the citizens of the capital.

Keys can be bought or made by combining Shackle with Iron Ore (one of them is also hidden in a chest in the castle prison).