Jeralt requests that Byleth read a book in his study in order to learn about Battalions. Completing this will unlock the feature for battle.
"Starting with this month's mission, you can now lead a battalion into battle. Search for the Tactics Primer in the captain's quarters and peruse it to get up to speed."
Quest-Giver Dialogue 
"A real battle this month, huh? I hear even the Knights of Seiros are involved. I never taught you much about tactics, but there's a primer on it in my room. Give it a read."
Quest Completion Dialogue
"I chose some knights to help you out. They'll be deployed in your next real battle. I'm sure you'll be able to make good use of them."
Given to you by Jeralt located in the Dormitory - 1F
- The ability to use Battalions
- Jeralt's Mercenaries
- Church of Seiros Soldiers
- Seiros Mercenaries
- 300 Renown
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.
This quest could throw some new players off with a glowing marker that's easy to glean past, but the tactics primer is located in the Captain's Quarters on the Second Floor of the Monastery in Second Floor Occupants. You'll see a bright yellow glowing marker on your minimap signifying it's location and a blue glowing spot in his room signifying the item in-game. Just press A to pick up the item and finish the quest. Easy enough.