Pitch-Black Ten

Pitch-Black Ten (漆黒の天, Shikkoku no Ten) is a manga by Kenshiro Sakamoto. It was serialized in the manga magazine Shounen Magazine R from October 2015 to April 2017. It is complete in three volumes. Kodansha Comics has licensed it for digital release in English.

Plot
One day, a cross comes down from the sky to land in ancient Japan. The cross brings with it disciples of the Lunar Religion called Missionaries, who preach that happiness can only be found in death and begin to slaughter anyone in their way. Meanwhile, in a small village, a girl named Momo works as a school teacher when a mysterious boy shows up at her feet one day. He has no name nor a family, so Momo names him “Ten” and invites him into her home to live with her. But what will happen when the shadow of the Missionaries falls on her once peaceful village?

Official
Official English site