In order to begin this mission, you have to go to the mine in Raven Rock and descent to its lowest level. It can be accessed in one way only - by helping Crescius Caerellius in The Final Descent. During this mission you will obtain one of the Black Books, crucial for the plot of the expansion. The mysterious volume will take you to the realm of Apocrypha, ruled by the crazy Daedric god - Hermaeus Mora.
After arriving in the dark dimension of the mad god, you should see a bridge and a path to the very top in front of you. You however shouldn't hope for an easy passage - if you don't hurry, you will be ripped apart by the tentacles. On your way you will be also attacked by a Seeker - the current mission will activate then. Kill the enemy and head towards hill.
Learn the Black Book's hidden knowledge
Go past the pond with a tentacle and take the Soul Gems from the pedestal. Afterwards turn on the switch and enter the rather uncomplicated labyrinth. The thing's easy - you need to move onwards (at best collecting more Soul Gems while you're at it) and turn on the switches you pass by. A Seeker will stand in your way, but you should be fine if you avoid the tentacle pond. Remember to heal up and prepare for a fight before entering the last room of the labyrinth. You will have to face a powerful Lurker, who's not an easy enemy.
During the fight with the Lurker, try to lead out as many powerful attacks as possible. At first you can also fire some surprise projectiles - the enemy should lose some health before figuring out who's attacking him. Sooner or later however, you will have to fight him directly - at this point a couple health potions will come in handy, as the beast can use some really deadly skills.
With the monster dead, approach the switch on the right and push it. That way you will open the door on the left (behind the door). Collect the valuables found there (including a few Black Soul Gems!) and use the next switch. Additionally, if you read the Worlds and philosophy which lies nearby, you will be able to learn a few new tricks with two-handed weapons.
Head to the newly unlocked passage and take a look inside the container there. Afterwards open the mysterious book lying on the pedestal. At this point the mission will end and you will have to make a decision regarding the desired power:
Reading skill books gives you an extra skill point
Your attacks, shouts, and destruction spells do no damage to your followers when in combat
Do 10% more damage and get 10% better prices from people of the opposite sex
After making a choice, you will be moved back to the mines of Caerellius and will be able to continue your journey.