Shingeki no Kyojin Gaiden: Kuinaki Sentaku

The young Erwin Smith is a rising star in the Survey Corps, humanity's only hope of defeating the man-eating monsters known as Titans. Ruthless and dispassionate, Erwin's mind is devoted to strategies and intrigue. But beneath Erwin's feet is another world, the Underground, where humans are born and die surrounded by the garbage the Capital throws away. Here, the criminal Levi survives on his wits and agility. But when these two ambitious men cross paths, who will prove himself stronger?

(Source: Kodansha USA)

Note: Includes the prologue and 2 extra chapters.