The following chapter provides a detailed description of the gameplay during The Heart of the Nora. You will find tips that can help you complete the quest in the most effective way possible.
The quest will begin right after your conversation with Sylens, which is also the final point of The Terror of The Sun. Head to Nora and go directly to Mother's Watch to continue your task. Technically, you can use fast travel option only to one of the bonfires, located close to the Embrace. You will have to travel the rest of the distance on foot or by horse.
Right before the very entrance, you will notice that Embrace is ruined. There are corpses lying on the ground, and machines and cultists scattered all around the area. Try to sneak behind them, as fighting all of them makes no sense. If necessary, leave the main road and head for the forests - bushes are the only available shelter.
In the end, you will reach a damaged Mother's Watch palisade. There are dozens of cultists and two Corrupters inside. They will notice you immediately, so a direct attack is the best option. Set them on fire. Finish off burning Corrupters with basic arrows. It's best to lure the machines outside the camp.
Finish the fight and follow the road. You will come across a small group of invaders but you shouldn't have any problems with them. You will reach an entrance to the inner part of the mountain. There is a corrupted Thunderjaw and several cultists. Furthermore, there is a Deathbringer Gun in sight. Pick it up or get closer to your enemies in order to continue your task.
After a short cut scene, you will see Nora running out of their hideout and facing the cultists. If you picked up the rifle, this should be a piece of cake. First, try to shoot down one of the disc launcher, and use the remaining ammo to hit the machine on its head. Near the starting point, you will find another Gun. Pick it up as soon as you run out of ammo and finish off your opponent. Now you need to take care of the remaining cultists. Do it and speak with Varl.
Go inside the mountain once the dialogue is over. Talk to Teersa.
The short cut scene will present Aloy entering the mountain. Follow the indicator but try to search all the chambers on your way and investigate all the notes. You will end up in one of the main chambers and watch a very important cut scene.
Regain control over Aloy and follow the indicator. Enter the elevator shaft and use the handles to climb up. Head to the exit.
This will trigger another cut scene. Wait for it to finish and your quest will be completed. A new mission will appear in your journal - The Mountain that Fell.