One of the many quests on the road to unlocking it requires you to collect Artifacts for Tarana.
Here's where to find Tarana and where the Artifacts are located.
Where To Find The Thief
After you've complete the Artifacts quest, you can move on to the next part of the Spire quests.
You'll need to head over to Colossal Corps, which is to the north-east of Boney Burbs and The Spire.
The Thief is actually Raz!
He can be found in the main building highlighted above.
Speak to him in the same way you speak to Tarana and you'll have completed this part of the quests.
Play The Last Log And Return to Raz
This next part is DEAD easy, as you just need to locate the Audio Log.
The audio log is located behind you on a brick wall, interact with it and then return to where you first spoke to Raz.
Talk to him and you'll have completed the quest, whilst starting a new one.