Combat Manual (秘伝書 lit. Secret Book) is a gameplay element in Fire Emblem Heroes introduced in version 2.9.0. Heroes are now convertible into Combat Manuals, which can be of various uses and help saving up Barracks space without sending heroes home or spending Orbs to expand Barracks.
In addition to saving up Barracks space, a Combat Manual has other uses that functions similarly to a hero, but with limitations. These uses include: inherting skills, merging and exchanging for Hero Feathers. In inherting skills and merging, Combat Manuals can only act as the source for such uses. Heroes converted into Combat Manuals are no longer able to participate in battle. A Combat Manual contains name, rarity, and the base skill set of the converted hero. It is important to note that merges, inherited skills, supports and blessings will be lost after the conversion. A Combat Manual can be upgraded in an identical manner to the Unlock Potential feature of a hero.
A Combat Manual can be used instead of sacrificing a hero to transfer skills from one hero to another and functions similarly to how the inheriting process works.
A Combat Manual can also be used for merging heroes and functions similarly to how the merging process works.
Finally, a Combat Manual can be exchanged for Hero Feathers if the manual of the converted hero has no skill that the player desires. The amount of feathers depends on the manual's rarity, in other words, matching the amount as if the converted Hero was sent home.