Either
get/build ffmpeg binary with 10 bit x265 support or do a 3 step process:
Code:
ffmpeg.exe -y -hwaccel auto -ss 00:26:45 -t 10 -i "vid.mp4" -map 0:v:0 -pix_fmt yuv420p10le -f yuv4mpegpipe - | x265 --preset medium --input - --output-depth 10 --y4m --crf 18.00 "vid(10bit).265"
ffmpeg.exe -y -ss 00:26:45 -t 10 -i "vid.mp4" -map 0:a:0 -c:a aac -ar 48k -b:a 210k audio.m4a
mp4box -add "vid(10bit).265" -add "audio.m4a" -new "output.mp4"