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This particular Daedric quest requires you to be at least level 21 to begin it. In addition to a level requirement, you must also have completed all of the other fourteen Daedric quests and have reached the
Blood of the Daedra portion of the game's main quest line so that you will have the chance to find an artifact and complete the quest - if you haven't already done so. Once you meet those requirements, you will be visited by a man named Casta Flavus the next time you rest. He will tell you that the Daedric Lord Hermaeus Mora himself has requested an audience with you, after which he'll mark the shrine's location on your map. If you're having trouble finding it, check the northwest corner of Cyrodiil (just to the west of the first "T" in "The Jerall Mountains").
The way to the shrine is long and treacherous, with a handful of opponents greeting you along the way. You don't need an offering when you finally reach the shrine at the top of a snow-covered mountain. Simply approach the shrine and Hermaeus Mora will ask a service of you. His followers are in need of ten souls - one from each of Cyrodiil's ten races. To capture these souls, Hermaeus Mora teaches you the
Mora's Soul Trap spell and hands you the
Hermaeus Mora's Soul Gem. When you've returned with the souls he has requested, Hermaeus promises to "further your journey on your true path."
In order to capture these ten souls, you're going to have to murder ten of Cyrodiil's citizens. This isn't really a difficult task, but you want to make sure that you don't kill anyone that might have some importance later in the game. If you haven't signed up to be a combatant in
The Arena yet, then that's a great way to get the souls you need. You're going to be killing each of the ten races as you move up in rank, anyway. Since you've already completed all of the other Daedric quests, another good way of avoiding any future problems is to revisit the previous shrines and kill their followers until you've acquired all ten souls. It seems wrong since you're actually doing a Daedric quest, but Hermaeus Mora doesn't care, and you won't need a blessing from any of the other Daedric gods for the rest of the game.
Once you have all ten souls, return to Hermaeus Mora and tell him the (good?) news. He's very pleased at your success and rewards you with the
Oghma Infinium, a book with great knowledge recorded within. Using the book gives you the option of choosing the Path of Steel, Path of Shadow, or Path of Spirit. Each of these paths corresponds to a class type, so choose the path that correctly fits your character. Your character's attributes and skills that are best affiliated with the chosen path will be boosted permanently by 10 points. Congratulations on finishing all fifteen Daedric quests!