AOMedia Video 1 (AV1) is an open, royalty-free video coding format designed for video transmissions over the Internet. It was developed as a successor to VP9 by the Alliance for Open Media (AOMedia), a consortium founded in 2015 that includes semiconductor firms, video on demand providers, video content producers, software development companies and web browser vendors.
Taken from Wikipedia (https://en.wikipedia.org/wiki/AV1)
Currently AV1 encoding is very slow compared to x265 encoder, However Intel release an Open Source AV1 coder called “SVT-AV1”.
Under my Testing, AV1 save more data compared with H.264 (x264 encoder) but not small as x265 encoded video
x265 slower preset 
SVT-AV1 Default
Both video leave audio untouched (Copy) and retain same attachment and subtitles, the video quality quite bad for AV1 at this time
SVT-AV1 encoding speed quite fast as x265 medium preset, I hope AV1 can compete same bitrate and highest quality
Download
SVT-AV1 can be download as IFME plugins: Download