Opus

Opusus (神書, ) are a rare type of book-resembling Material that can be won from clearing certain Battles in History Mode from only the DLC History Mode Maps, usually in Distortion Battles that can only be fought after clearing a History Mode map.

Opuses are essential to create Sanctuary Crests for a specified personal weapon, which will overwrite that personal weapon's True Power attribute, unlocked or not, with the Divine Favor attribute, which increases that weapon's Might to 720, as well as give them a pulsating neon glow on portions of their weapons while the player is controlling that Hero Unit.

Fire Emblem Awakening DLC
=Trivia=
 * Peculiarly, there is a inconsistency with Oboro's Opus. It is labeled "Oboro's Lance's Opus", despite both the weapon and Oboro's Scroll being labeled "Oboro's Spear".