libav was the fork [1], and libav is dead now [2,3] (and many developers returned to ffmpeg).

libav was forced to rename their project because Fabrice Bellard, still owner of the trademark and the ffmpeg.org domain, did not hand it over. ffmpeg put in much effort retain compatibility with libav, mostly by merging all their changes wholesale, while libav ignored ffmpeg, including and most crucially, their security fixes [4].

[1] http://blog.pkh.me/p/13-the-ffmpeg-libav-situation.html

[2] https://www.libav.org/

[3] https://www.phoronix.com/news/FFmpeg-Back-In-Debian

[4] https://j00ru.vexillium.org/2014/01/ffmpeg-and-the-tale-of-a...