You will come across the Thief and the Merchant towards the end of the Catacombs. There are a pair of prisoners trapped in adjacent cells. Walk over and speak to one of them to learn of their predicament. Essentially we need to choose who lives and who dies and depending on your choice you will get a different reward. Saving the fellow closest to the stairs rewards you with a Trinket (Magic Leech) saving the other, will earn you the Trinket (Dim Hope) .
Note: if you save the fellow for the Dim Hope Trinket, you will find that a side-quest giver in the Citadel later on will be dead.