Attack on Titan Key

When man eating Titans first appeared 100 years ago, humans found safety behind massive walls that stopped the giants in their tracks. But the safety they have had for so long is threatened when a colossal Titan smashes through the barriers, causing a flood of the giants into what had been the human's safe zone. During the carnage that follows, soldier Eren Jaeger sees one of the creatures devour his mother, which leads him to vow that he will kill every Titan. He enlists some friends who survived to help him, and that group is humanity's last hope for avoiding extinction at the hands of the monsters.

Eren's father gave him a key and told him to embrace the power of Titan transformation if he wanted to protect Mikasa and Armin. His father said that the key is for the basement under their home in Wall Maria where all the secrets would be revealed.