It's about twins who were separated at birth. Matin (the one with the long dress) was crowned as the empress of a kingdom. Nox (the one with the braid) was the actual heir to the throne, but an organization called Agiona separated him away from Matin since they were controllers of the kingdom. Matin held the holy sword, Trisagion, so Agiona thought that it would be best if the wielder of Trisagion should rule the kingdom. He was taken away by a man named Adhuk, was raised as his son, and he also had two more siblings. However, one of Nox's adoptive siblings was killed by soldiers from the kingdom (as shown in 1:32), causing Nox to hate the kingdom (unaware that it was his real homeplace) and build an anti-alliance against it. Matin drove her army to war and soon the twins would start fighting each other. It wasn't revealed in this video, but Matin knew Nox was her brother, while Nox didn't know Matin was his sister. Matin knew Nox hated her, but she still loved him and wanted to save him from his hatred. During the war, she put that thought aside and fought Nox.