Arms Scroll is an item that gives the user a weapon level up according to what weapon was currently equipped. While doing this, caution is required seeing as you may accidentally upgrade a weapon level you had no intention of upgrading simply because the wrong type of weapon was equipped at the time of use. In Radiant Dawn, they are often found hidden in outdoor levels on the ground. They first appeared in Fire Emblem: Path of Radiance, and have appeared in every game ever since. In Fire Emblem: Radiant Dawn, 5 are sold at the base shop for the Part 4 Endgame if it is necessary to raise the weapon levels of some of your best units. In Fire Emblem: Shadow Dragon, there are four Arms Scrolls in the entire game. One is found in a chest in Chapter 14. The other three are available in a secret shop in Chapter 24.
In Fire Emblem: Awakening, 
Arms Scrolls grant an immediate weapon level up to all weapons the character is capable of equipping - a Tactician will gain one level in Tomes and Swords, while a General will gain a level in Axes and Lances.