The song is freaking awesome, and the chorus is one of their best in a long time. It is a rock song. As one can probably tell from the cover, it is a darker song than their other recent singles (Heavenly Blue, Believe, especially Ring Your Bell). You can see the TV commercial for it here.
If you did not know already, this song is the second ending theme for the anime Arslan Senki (The Heroic Legend of Arslan in English), and is succeeding Eir Aoi’s Lapis Lazuli as the ending theme (which is already an awesome song in itself). The release date for this single is August 12, 2015. The TV size video is here.
(For regular edition)
1. One Light
2. Mahiru (真昼)
3. Samidare ga Sugita Koro ni (五月雨が過ぎた頃に)
4. One Light ~instrumental~
EDIT: Short version of the PV here.