After getting your reading from Olava, you will have to locate the dead assassin. The location you want to travel to is the "Deepwood Redoubt", which is just west of the small town of Dragon Bridge (you have to travel up into the mountains to find it). The location itself will be guarded by some Forsworn enemies that will attack if you get too close. Kill them off and enter the Deepwood Redoubt.
In here, you'll run into MORE Forsworn enemies. Beware here though, because there are a LOT of traps. For example, the chest on the right is trapped, there are FLOOR SWITCHES everywhere (look out for blood and raised floor plates). Follow the path (past the axe trap) until you reach a LOCKED EXPERT DOOR. Now you can either pick it or go kill another Forsword enemy in the next room for the key (beware of the enchanted snow traps here). The Forsworn with the key is a powerful magic user though, so be careful!
Unlock the door and watch out for another floor switch trap (this one springing a wall of spikes your way). Ahead is an iron door leading to the arch you've seen earlier (kill any archers here). Keep going, killing one more looter until you can zone into the "Deepwood Vale".
As you can guess, this new outside area (we are back out on the world map after all) is full of forsworn. Head forward into their camp where they have a mix of melee, archers, and some magic. Nothing really notable here except the SKILLBOOK, "Sacred Witness" (SNEAK) in one of the tents and the fact that one of the forsworn is a BRIARHEART (meaning he's tougher to kill). If you've gotten this far though you should have no problems. Go forward and discover "Hag's End" and then head inside the door.
Back inside, head left but be prepared for battle. Four witches are in this dining room. The three that say "witch" will fight to the death, but the fourth one will warp away upon taking damage! Coward! Loot the robes and explore/loot the place (some bedrooms are down here as well as a storage area with magic ice traps). Head up the nearby stairs to continue. At the top of the stairs is a trap floor plate, so dodge it and rob the chest. To the right is the coward witch with a regular one, so damage her until she runs away again. There's an ILLUSION SKILLBOOK here called "Mystery of Talara, Part 4" that you should be sure to grab.
Now, the BROTHERHOOD ASSASSIN is near. See the throne in this room? Go behind it and find a switch on the left-hand wall. This lowers the rock door nearby. Inside is the assassin. Just seeing him technically completes this quest, but just LOOK at his armor:
Ancient Shrouded Boots: Wearer is muffled and moves silently.
Ancient Shrouded Armor: Increase poison resistance by 100%.
Ancient Shrouded Gloves: Double sneak attack damage with one-handed weapons.
Ancient Shrouded Cowl: Bows do 35% more damage.
Wow, very nice. Ok, normally I'd say we're done, but we gotta catch that cowardly witch...
Lower the ramp to the right and continue. Kill the witch that comes out of the room on the left, and then go in. There is a FROSTBITE SPIDER in here that will try to be sneaky and ambush you from above, so watch out. There's also a "Hag's End" key here too on a brick jutting out of the wall. Be sure to find the key as it will help out a lot. For example, continue on and to the right is a MASTER locked door the key can unlock (the chest here triggers a flame trap). Activate the chain to the right and wait for the madness to stop, then go up and kill all the witches and watch the coward run again.
Loot the room (there's several doors the key unlocks in here) then pull the chain in the middle of the room to open a iron gate. Go into this area and there's a DRAGON WALL on the right that teaches the word SLOW TIME - TIME. A useful DRAGON SHOUT.
Head outside now for a final showdown. The witch and two Ice Essence enemies will come out. Be sure to dodge and move here (don't be a sitting target) and take them out (heal when you must, her fireball hurts). She won't run this time so you can finish her. There is a special dagger on the table here as well as a chest you can loot. That finishes this quest.