Complex sentence: Ame no Murakumo and its priests set this arrangement up to test the powers and determination of the future Miko of Sun and Moon, who must break the seal to awaken the god of swords.
Split into simpler sentences: Ame no Murakumo and its priests set this arrangement up themselves to test the powers and determination of the future Miko of Sun and Moon. The Miko must break the seal to awaken the god of swords.