Into The Ground (狭間へ沈みゆく, Hazama e shizumi yuku Sinking into the Ravine in the Japanese version) is Chapter 6 of Fire Emblem Fates in the Revelation Version. The Birthright-equivalent chapter is In the White Light with the Conquest-equivalent chapter is Embrace the Dark. This chapter takes place in the Plains of Hoshido.
Corrin chooses not to side with Ryoma. Xander commends them, but they say they are not siding with Xander either. They say they cannot choose between them as they don't want to betray either side of the family. They plead the two to put their weapons down and find a solution together. Xander and Ryoma ignore Corrin's words and move to engage each other. A battle begins with Corrin overlooking the siblings fight. Azura remains with them, as their servant (Felicia/Jakob) rushes to join their side.
Wanting to catch their attention, Corrin, Azura, and their servant decide to take down the forward commanders of both armies and succeed. Unfortunately, both Xander and Ryoma view Corrin's actions are a deliberate act of betrayal. Corrin, Azura, and their servant are labeled as traitors to be arrested immediately. Unable to convince either army to listen, Azura pulls Corrin away from the battlefield into a nearby forest.
Needing a safe place to talk, Lilith then takes Corrin to their special castle so they can discuss with Azura what their plan of action is. Azura tells them that she has something important to tell them, but will have to go to the Bottomless Canyon before she can reveal it. Corrin agrees to follow her, having no other options.
The player simply needs to defeat the two enemy commanders within the turn limit in order to complete the map.
The script for this chapter can be found here.
Subjective: The following part of this article is subjective to the editor's writing. Therefore, it may not work for everyone.
The objective is to defeat both enemy commanders within 5 turns.
There are 3 enemy units on each side (excluding the four siblings on each side, as they do not take part in the battle) on Normal, 4 on Hard, and 5 on Lunatic. You will have control of Corrin, Felicia/Jakob, and Azura.
On Normal and Hard, the Nohrian troops are Wyvern Riders, and will immediately fly over. The lead siblings (Xander/Ryoma) will not attack unless you enter their range. Once you've killed the enemy commander, Ryoma will tell the other Hoshidan units to "hold their positions," so they will no longer attack your units. Note that even after you defeat the enemy commander, if you enter Ryoma's range, he will attack. The same goes for Xander and the Nohrians.
On Lunatic, the Nohrian troops are replaced with other, non-flying enemies, and will not move until you enter their range. To get to them, have Corrin walk on the river, otherwise you won't complete the mission in time. Use Azura and Felicia/Jakob to deal with the Hoshidan soldiers, and equip Corrin with the Dragonstone when he's in range of the Nohrian soldiers, so he can survive their attacks.