Learn the Black Book's hidden knowledge
Notice! This mission will become available after completing Unearthed!
In the last room of Kolbjorn Barrow you will find the Black Book - the real gate to the realm of Apocrypha. Pick it up and you will be moved to the land of Hermaeus Mora. Strange rules are in force here - you're safe only within the light. Whenever you leave the light, you will start receiving damage. Note that the only source of light are the strange fireflies. Follow them to reach the gate behind the stairs. Alternatively, you can use a torch or another source of light - they work just as well.
You will reach a long, dark corridor. On its sides there are dangerous tentacles and in the middle a few Seekers and a Lurker. Kill the enemies from a distance (remember to avoid the dark areas!) and once they're down, run through the tunnel in front of you. After getting to the end, you will reach a illuminated pedestal where you will have to fight with a Lurker. Try not to leave the light and defeat the monster as fast as possible (don't enter the pond from which its came, it's highly dangerous!).
With the monster dead, approach the shelf with the Black Book. Open it to face an uneasy choice regarding a new power:
Secret of Strength
Power attacks cost no Stamina for 30 seconds
Secret of Arcana
Spells cost no magicka for 30 seconds
Secret of Protection
You take half damage for 30 seconds
After deciding on one of the options, use the book once again and return to Kolbjorn Barrow. If you ever want to change your choice, just return to Apocrypha (by activating Filament and Filigree).