From Halopedia, the Halo wiki
Overrun is an achievement in Halo: Spartan Assault. It is awarded when the player completes the first campaign level, Operation A: Umbra. The achievement is worth 10 Gamerscore on Windows systems and Xbox 360, and 20 Gamerscore on Xbox One. It is represented by a circle with an "A" inside it.
If previously unlocked on another platform, this achievement will unlock on the Xbox One the first time the player signs in. On all other platforms, players must play through the campaign again to unlock the achievement.