Attack on Titan: Before the Fall (Manga)

Shingeki no Kyojin: Before the Fall is a currently ongoing manga by Satoshi Shiki, based on the light novels by Ryou Suzukaze. This manga serves as an official prequel to main story by Hajime Isayama. There's currently one single prologue chapter published, with the first chapter coming in the next issue of the Shonen Sirius magazine.

Plot
Seventy years before the fall of Wall Maria, a Titan appeared inside. People were instructed to keep this incident a secret, however.