Hyakuren no Haou to Seiyaku no Valkyria

Japanese Title: 百錬の覇王と聖約の戦乙女〈ヴァルキュリア〉

English Title: The Master of Ragnarok & Blesser of Einherjar

Plot
Yuuto Suoh is a normal high school student who is knowledgeable about urban legends. In order to prove one of these legends, he and his childhood friend Mitsuki Shimoya visit a local shrine nearby. However, while using his solar-powered smartphone to take a picture of himself next to the shrine's divine mirror, he gets mysteriously transported to another world called Yggdrasil.

This new world he arrives in is heavily steeped in Norse mythology. He also finds that many people fight each other over land domination. Yuuto is later taken in by the so-called Wolf Clan, a group of people with weak military tactics. Using the knowledge of military tactics he gained from school and his smartphone, he gradually makes the Wolf Clan rise to prominence. In turn, he also gains the affection of a group of magic-wielding warrior maidens known as the Einherjar.