The Imperial Trench represented a scar on Ilum where the Galactic Empire had carved out a significant section of the planet to facilitate their mining efforts focused on extracting the planet's kyber crystals. By 14 BBY, this excavation was readily observable from orbit.
Subsequently, the First Order utilized the trench as the location for the Starkiller Base superweapon.
The first appearance of the Imperial Trench was in Star Wars: Episode VII The Force Awakens. Its identity was later confirmed in the 2019 video game Star Wars Jedi: Fallen Order.