New main objective: Read the Elder Scroll at the Time-Wound
Before you can read the Elder Scroll, you have to leave the Tower of Mzark. You luckily don't have to cross Blackreach and the underground ruins again, as you can use the lift in the eastern part of the tower (screen above). Interact with the lever and wait until you're transported to the surface. There, pull another level that previously blocked access to the elevator from the outside.
Note: Before you decide to continue this quest, be sure to stock on potions and raise your character's fire resistance. Afterwards open the world map and use fast travel to reach The Throat of the World. Listen out to a new conversation with Paarthurnax and stand inside the Time-Wound. Open the inventory, choose the Elder Scroll and press E to read it .
New main objective: Learn the "Dragonrend" Shout from the Nord heroes
A longer cinematic will start, during which you will only be able to look around. You will witness the events from the past, as the Nord heroes managed to bring Alduin to the ground and accidentally sent him to the future. The most important part of the cinematic will be of course learning the three new Words of Power (screen above), i.e. Word of Power - Mortal, Dragonrend, Word of Power - Finite, Dragonrend and Word of Power - Temporary, Dragonrend. Therefore, you will unlock the Dragonrend Shout.