This chapter is a complete guide for The Sun Shall Fall quest. Here you can find tips that will help you to finish this quest in the most beneficial way.
The mission appears in your journal after you finish the previous one connected to Erend - Into the Borderlands. To start this quest, go to the Palace in Meridian. When you get there, speak with Avad. After a long dialog the mission will be updated.
Go to the place pointed by the marker. When you get there, a short cut-scene will play. After that, get closer to the bomb left by Dervahl. Next, go to the floor and investigate the surroundings by using Focus. Besides the barrels you can also find the handle presented on the picture above. Interacting with it allows you to push the barrels through the window. You should do that.
There will be an explosion. Erend goes to the Palace and your task is to investigate the yard around the burned house. You quickly come across tracks that lead to the city center. Follow them. The tracks lead you to one of Meridian houses. Go to the building's basement and you will see that there is a passage between the city and the Palace terrains. Go through the secret passage.
When you are close to the climbing wall, catch the handle presented on the picture. Go down and left, Aloy will catch the right handles. Start climbing when you are closer to the bridge, you quickly get to the tunnel that goes under the bridge's main passage. Go forward, you will come across two mercenaries. Kill them and use stairs to get to Palace's upper level.
There, you have to watch a short cut-scene. After that, you must destroy the device that Dervahl used to stun Erend and Avad. Next, go after Dervahl and face him and his men.
The main enemy uses a powerful cannon that emits waves that hurts and pushes Aloy back. Shoot him with a few arrows, use a lot of dodges and try to eliminate his companions first. Dervahl is also vulnerable to melee attacks but be careful, he can also deal a lot of damage if you get closer.
When you defeat him it turns out that he called machines to help him. You must eliminate Glinthawks. Set them on fire by using a few arrows at the same time or use single ones. When you deal with one wave, you must prepare to face another. In total, you must defeat seven Glinthawks. After that, speak with Dervahl.
A short cut-scene awaits you and after that you can speak with everyone involved in this quest. After speaking with the last one this mission and Erend's quest line are finished.