At the Summit of Apocrypha (quest)

Unlock your "Bend Will" Shout
Before trying to face off with Miraak, unlock the Bend Will shout using a Dragon Soul.

Read "Waking Dreams"
Re-read Black Book: Waking Dreams to travel the part of Apocrypha where Miraak has holed up.

Reach Miraak's Temple
Pass through the maze of chapters ...

Use the "Bend Will" Shout to tame Sahrotaar
... until reaching an apparent dead end. Here use the Bend Will shout to tame the dragon Sahrotaar. It will land letting the Dragonborn ride it. Travel past a small island in the slime, taking out a few Lurkers, until reaching the temple.

Defeat Miraak
Take on Miraak. There are three dragons up here, and Miraak's health will need to drop four times, since three times he will absorb a Dragon Soul upping his health again.

On Miraak's body a full armour set plus a sword and a staff can be collected. Opening the black book on the pedestal now, lets the Dragonborn choose any skill to harvest all the perks invested, by sacrificing a Dragon Soul. This can be done any number of times, and also when revisiting the book later in the game, though the Dragonborn does need to be on Solstheim.

Thus ends the Dragonborn DLC main quest line.

Auf dem Gipfel von Apocrypha (Aufgabe)