Against Beatmap Editor is a free tool that allows you to create your own maps using your own music. Using this editor you can place enemies, weapons and obstacles in perfect synchronization with your music. Then you can play your custom maps in AGAINST.
The app has been added to your Steam library. If you can't find it, make sure that your selected search filter allows applications.
https://store.steampowered.com/app/1853600/AGAINST_Beatmap_Editor/
If you need help with the Editor, watch an in-app tutorial or join a community-driven Discord server for AGAINST mapping and modding.