Skyrim:Locate the Assassin of Old

Quick Walkthrough
#Speak with Olava the Feeble.
#Travel to Deepwood Redoubt.
#Make your way to Hag's End.
#Discover a Dark Brotherhood Assassin behind a hidden wall.
#(Optional) Defeat the hagraven.

Detailed Walkthrough
The Reading
Upon successful completion of the Dark Brotherhood quest Breaching Security, Gabriella will hand you Olava's Token as a special bonus, if you managed to slay Gaius Maro in any hold capital. According to Gabriella, this is an opportunity not to be missed, so travel to Whiterun and locate the elderly seer Olava the Feeble. She can usually be found relaxing on her porch behind Breezehome. Show her the token and a surprised Olava will immediately realize why you are there and will then reluctantly provide a reading of your future. Along with some predictions about the Brotherhood, Olava will suddenly see a potential for adventure and wealth, a place hidden behind Deepwood Redoubt, namely Hag's End. The last resting place for an assassin of old, his fortunes are yours to take. A map marker will be added and an exhausted Olava will send you away.

Deepwood Redoubt
Like most other ruins in The Reach, Deepwood Redoubt is crawling with Forsworn enemies, so clear the exterior and enter. Watch out for the dart trap right after the first catacomb, then dodge the archery attacks from the Forsworn enemies overlooking the first chamber. Explore the stony steps to the right of the first set of stairs to find a dead adventurer and a locked chest. Move quickly through the southwestern tunnel, or an enemy may trigger the swinging blades trap. Proceed to the first room and avoid the elemental runes on the floor. Taking the path to the right will lead to a pair of rooms with an Arcane Enchanter in one, and a Forsworn enemy in the other. A Deepwood Redoubt Key is carried by the Forsworn, which unlocks the southeastern door in the room with the runes. Another trap awaits, so avoid the pressure plate, kill the last Forsworn enemy and enter Deepwood Vale through the door.

Deepwood Vale
As suggested by the name, Deepwood Vale is an isolated outdoor valley, and can only be accessed through the previous cave. Follow the path down to the bridge and then if not attacked drop into the river and follow it right to an apprentice-locked chest and a Nirnroot under the waterfall. Also there is a sneaky master-locked side-door entrance at the base of the Southern right-hand side tower. After the unavoidable fight against another wave of Forsworn, explore the area for some minor loot inside the southern and northern towers, and a hidden area on top of the waterfall including an expert-locked chest and a pair of fixed  Predator's Grace boots. There is also Quicksilver ore vein here. Finally, locate the entrance to Hag's End and enter.

Hag's End Horror
Immediately after opening the first iron door, you will encounter several hostile witches, along with a hagraven. As soon as you attempt to fight the hagraven, she'll teleport herself deeper into the dungeon unless you kill her first, so there is not much to do but pick up the chase. Look to your left as you enter the room for an alchemy lab and a Scroll of Firebolt. Watch out for several traps in the tunnel ahead, then enter the next room for your second encounter with the hagraven and a few witches. Pick the fight, watch the hagraven disappear, then locate the handle behind the throne on the left hand side to lower the stone wall. Your target, the corpse of a Dark Brotherhood Assassin, is hidden behind the wall. He is clad in a complete, unique set of Ancient Shrouded Armor, and by his side is an unlocked chest with leveled gold and loot. When you are done, locate the lever to lower the bridge.

Finishing the Hag
Although optional, you may continue on to clear the ruin. Cross the bridge and check the left room for a frostbite spider and the quest-related book The Legend of Red Eagle.  This room also contains a copy of the Hag's End Key which is easily missed on a small shelf to the right of the fireplace. This key unlocks the master-locked gate ahead, which contains a locked, trapped chest. Activate the nearby handle which will elevate three sets of gates and shoot spikes out of the wall. The hagraven is back in the next room, along with two witches and a huge pool of oil. Dispose of the witches, attack the hagraven until she disappears, and pull the chain. This will close the path behind you, but open up for progress on the eastern wall. The next chamber is empty, but contains a word wall for part of the Slow Time shout. Finally, open the door for your final encounter with the hagraven, and your additional rewards. 

You will find yourself at the very top of Deepwood Vale, with the hagraven waiting for you on a ledge. When you approach the nearby shrine, two leveled creatures will appear and attack. If you managed to kill her before she teleported, her corpse will appear on the ledge and no creatures will attack. Fight the opponents while dodging the destruction spells from the hagraven. Use caution here as you could easily fall to the lower level and have to re-enter the door and work your way back up to the ledge. If you didn't grab the key from the Frostbite Spider room, the hagraven carries another copy. The unique dagger Bloodthorn can be found on the altar, and a boss-leveled chest sits to the left of the door you last came through.

Notes
The switch which lowers the stone wall to reveal the Assassin of Old will not appear on the wall behind the throne if the player has not received Olava's reading.
You can keep Olava's Token if you drop it or put it in a container while she is still talking. Then just wait until she is finished and the quest updates, and now you can pick up the token and keep for an everlasting memory of the dear old lady or sell it for profit.
It's possible to climb your way to the altar and pickup Bloodthorn before you enter Hag's End - from the secret area above the waterfall, work your way around the rocks.
The hagraven's teleportation is scripted, and will always occur if you attempt to attack it using any means. However, it is possible to kill the hagraven before she teleports if wielding Mehrunes' Razor. If you manage to kill her before she teleports, she will appear dead at the top of Deepwood Vale.
You can get the reading from Olava the Feeble at any time after completing Breaching Security, but the predictions will remain the same - even if you have completed the Dark Brotherhood questline and they have already happened.
It is possible to obtain the Ancient Shrouded Armor without Olava's token if you've destroyed the Dark Brotherhood. Take Serana with you into Hag's End and during the fight with the first set of witches, she will resurrect the body of the Dark Brotherhood Assassin, who will join you. When he turns to ash, loot the ash pile for the entire set of Ancient Shrouded Armor. Another option without Serana is to have the Ritual Stone activated. In the room where the second encounter with the hagraven occurs, stand as close as possible to the stone wall behind the throne and active the power of The Ritual Stone. Exit the area and the corpse of the Dark Brotherhood Assassin will be reanimated and following you. Kill him and to loot the armor from the remains.

Bugs
  
 
 
 You can summon a living version of the assassin to get his armor with   He will attack you though.
 
 
 
 

Quest Stages