Naia is a framework/code and resources library for the Battle for Wesnoth add-ons developed under the Project Ethea organization. As of this writing, this includes the Invasion from the Unknown and After the Storm campaigns.
This is not a Wesnoth add-on and it will not do anything on its own if installed. It is, however, a hard dependency for IftU and AtS. When installing the campaigns from the Git repositories (as opposed to using the versions from the official add-ons server) you have the choice of installing Naia inside each campaign’s directory (not recommended, but this is used for official distribution), or installing it beside the campaigns in the Wesnoth add-ons directory. The latter method is especially useful if you plan on keeping both campaigns around.
Do note that due to the shared nature of Naia, changes in it may break either campaign and this may not always be addressed immediately. In particular, it is not advisable to use a version of Naia other than the one shipped when playing release versions of the campaigns.
Naia presently requires Wesnoth 1.14. Versions 1.14.2 and later are recommended, and it may also work with versions 1.13.12 through 1.13.14, albeit you will not receive any support for attempting to use those versions.
If you encounter any issues with a campaign that depends on Naia, you must report them on the campaign’s development topic on the Wesnoth.org forums. Alternatively, if you are 100% positive that the issue lies within Naia, you may consider using its own bug tracker instead.
Be aware that depending on the nature of the problem, you may be asked to provide a saved game file to reproduce it.
You may contact the author of this campaign via forum PM to Iris on the Wesnoth.org forums.