Don't Download Unless You Are A Developer

Putting this in your Mods folder will do nothing.

Developer Description

This jar includes the source code and classes needed to interact with Distant Horizons in a Minecraft and mod loader independent way. This jar should be used during development instead of the full Distant Horizons jar to prevent accidentally using unsafe internal methods.

The example projects on GitLab contains all the basics you need to get started using the API.
https://gitlab.com/distant-horizons-team/distant-horizons-api-example

Additional resources

API JavaDocs
https://distant-horizons-team.gitlab.io/distant-horizons/

Developer Wiki
https://gitlab.com/distant-horizons-team/distant-horizons/-/wikis/home

Discord
In case you have any questions or issues not addressed in the above two documents.
https://discord.gg/xAB8G4cENx

Issues
If you find any bugs or want to request a new feature, for the API, please use the main Distant Horizons GitLab issue page.
https://gitlab.com/distant-horizons-team/distant-horizons/-/issues