Nohr (暗夜の同胞, lit. Dark Night Brethren) is Chapter 1 of Fire Emblem Fates.
The script for this Chapter can be found here.
Subjective: The following part of this article is based upon the editor's personal experiences and opinions, and therefore may not be applicable for all readers.
In this chapter, you learn the basic concepts of combat. The goal is to defeat Xander in battle. Xander retaliates back when you attack him, but he will not attack you during his turns. After you attack him once and his retaliation gravely wounds Corrin, he will suggest that you use the healing tile that he subsequently summons through use of the Dragon Vein ability. He will then order you to stay in the healing area until you are sufficiently healed enough, after which he will ask you to attack him again. Once he is defeated, the chapter is completed.
It is advisable to only attack Xander if you have more health than his attack states, as a defeat here will result in a game over. Since he does not retaliate or move during the enemy turn, you are free to heal up enough from the healing circle created from Xander's use of the Dragon Vein without fear of an attack from him.