NetRadiant is a game level editor for idTech-derivated games and the recommended level editor for Unvanquished mapping.
The NetRadiant project also provides the Q3map2 map compiler.
Other usable options are GtkRadiant and DarkRadiant (there may be more limitations), see Level editors for more information.
The tool is maintained by Team Xonotic (from the Xonotic game).
illwieckz is both maintainer of Unvanquished and maintainer of NetRadiant at Xonotic.
See Getting started with NetRadiant for installing it and getting started.
The NetRadiant website is netradiant.gitlab.io.
The NetRadiant source repository is gitlab.com/xonotic/netradiant.
The repository also provides source for the Q3map2 map compiler.
The source repository for the Unvanquished gamepack is github.com/Unvanquished/unvanquished-mapeditor-support
NetRadiant is a brush-based game level editor.
You'll get additional information about NetRadiant on the Level editors page.