Unlicensed, but publicly available on MCResourcePile

"Viridun does not have any license attached to it and has instead been sourced from a publicly available download published by the original author. ResourcePile strongly advises contacting the original author to obtain permission to use this map for anything other than private, personal use. Please be considerate when using this map and respect the original author."

https://mcresourcepile.github.io/maps/overcast, https://web.archive.org/web/20160919055203/https://oc.tc/forums/topics/505b7ecbb944ec559400150e