While the game's called Three Houses, there's actually an alternate fourth path you can take. Here's how to unlock the Fire Emblem Three Houses fourth route.
Guides Archive - Page 1026 of 2617 - GameRevolution
FIRE EMBLEM: THREE HOUSES - BLACK EAGLES 】 BE Blind - 2nd Route MADDENING NG+
Black Eagles [Fire Emblem Three Houses] : r/streetmoe
My Cannon which route would each character align with in the war : r/ FireEmblemThreeHouses
Guild Wars 2: Path of Fire - How to Access the New Story and Map - GameRevolution
Fire Emblem: 10 Facts About Three Houses's Black Eagles That You Didn't Know
FIRE EMBLEM: THREE HOUSES - BLACK EAGLES 】 BE Blind - 2nd Route MADDENING NG+
My Fire Emblem Three Houses Review (Black eagles route)
Fire Emblem: Three Houses - Faction Background and Character Spotlights Showcased in Black Eagles House Introduction Video - SAMURAI GAMERS
Fire Emblem Echoes: Shadows of Valentia How to Clear North Gate Battle (North Gate) (Act 3) - GameRevolution