Attack on Titan - Season 0 Episode 30 : ―Special Omnibus― 4th Night ～The Truth of the Titans and the Secret of the Walls～

Many years ago, the last remnants of humanity were forced to retreat behind the towering walls of a fortified city to escape the massive, man-eating Titans that roamed the land outside their fortress. Only the heroic members of the Scouting Legion dared to stray beyond the safety of the walls – but even those brave warriors seldom returned alive. Those within the city clung to the illusion of a peaceful existence until the day that dream was shattered, and their slim chance at survival was reduced to one horrifying choice: kill – or be devoured!