Once you have reached level 150, go to the Priestess Sebina in Devias to hear the story of the 'Scroll of Emperor'. You will need to offer a token of 1 Million Zen for the Priestess's story.
The 'Scroll of Emperor', which tells of Muren, the first emperor to unify MU, is said to have been seen around the Lost Tower, Atlans, and the Dungeon.
Legends have been told of the Scroll being enveloped by a great power and whoever reads from the scroll shall be able to wield this power.
Once you find the Scroll, go to the Priestess who will aid you in unlocking the Scroll's power.