When you first enter Blackcrag Fortress you'll find yourself in a small storage room with a bunch of crates and barrels, and a man standing against the northern wall. Speak to the man, being sure not to threaten him, and he'll give you information about where you can find Empress Prazac and tell you that the items in this room and the one to the south are for you.
Leave the room and enter the room just across the hall to the west. You'll find Howar standing in here. Howar sells alchemical recipes, and he also possesses the Grimoire of Then, which he'll give you once you've spoken to Shane about it.
To the south of Howar is Petrie. Petrie will give you a quest to bring her a Golem Diadem. For every Golem Diadem you bring you'll receive a pair of Boots of Speed and 250 Experience. I'm not sure if it's a bug that lets you get this quest numerous times, but it's a nice way to get the boots for each of your party members, and of course, the experience is always nice as well. Petrie also sells some Mage spells.
Slightly to the south and east of Petrie is Shane. Ask him about the Grimoire of Then and he'll tell you that he thinks the alchemist has a copy. This is Howar, so go and ask him about it to receive the book for Cerulean of Moon's quest. Shane sells a few items, is an unlimited items reseller and also identifies items.
The door leading south just to the west of Shane leads into the kitchen and dining area of Blackcrag Fortress. If you frighten the woman in here she'll tell you take anything you want, but since you can just pick it up anyway I just avoid talking to her altogether.
There's a secret passage in the southwest corner of the room which leads to a small area outside the walls. Head south down here and you'll be ambushed by some Empire troops that aren't too happy about the Empress agreeing to speak to you. When you get close to the end of the passage start checking the western wall for another secret passage.
This secret passage puts you in a room with a bunch of statues and a door leading north. Go through the door and you'll find yourself in the antechamber to the throne room. There's a storage room which is empty to the south, and the library is to the west. Enter the library and go to the southwestern corner. Read the book to learn Cloud of Blades Lv.3.
Now enter the storage room and check out the southern section of the western wall for another secret passage. This leads to a short passage going north/south, with several force barriers blocking it. Use Dispel Barrier Lv.2 or a Piercing Crystal to get through the barriers and check the western wall for a secret passage near the northern end.
This takes you into a small forge, which is run by Winn. She sells weapons, and also provides item augmentation for 5000 coins. Once you're done head back to the room with the statues.
There's a storage room which is empty to the south, and the library is to the west. Enter the library and go to the southwestern corner. Read the book to learn Cloud of Blades Lv.3.
Now head back into the main room and speak to the person standing near the door. This is Vladimir and you need his permission to speak to Empress Prazac. Just talk to him for a bit and he'll let you through.
The last room is the throne room, which contains Empress Prazac. Talk to her a little bit, and she'll eventually give you a quest to take a message to Anaximander in Fort Emergence. Accept the quest and, once you've completed it, you'll get permission to enter Footracer Province from the Empress.