Hag’s End is is a small ruin found within the Deepwood Vale outdoor area accessible via Deepwood Redoubt. Deepwood Redoubt is found in the mountains west of Dragon Bridge, slightly southwest of the barrow Volskygge.
A Hagraven leader will teleport away from you as you chase her through the dungeon, hurting her and killing her minions. Before you chase her out the door, don’t forget to grab the Shout for Slow Time from the Word Wall in the final chamber. After the Hagraven leader outside on the ledge notices you expect a couple Ice Wraiths to materialize behind you.
Once you’ve dispensed with the Ice Wraiths, try and work your way over to the waterfall (on the right facing out to the Vale). There is a chest with decent loot and pair of Predator’s Grace boots (although the Ancient Shrouded Boots are better).
The “assassin of old” is located in hidden room behind a throne on the second floor. To access it, use the switch right behind the throne and a section of the wall will slide down revealing the corpse and a chest. If you do not have the quest, however, the button will not exist.