I'd personally stick to video tools for generating video. Imagemagick is pretty awesome but it's no ffmpeg. ffmpeg (or avconv as it's now known) can handle both frame-stitching and fade options in one go:
ffmpeg -r 24 -b 500000 -i frame%05d.png -vf fade=out:155:45 output.mp4
If you're not familiar with ffmpeg, here's a quick translation of that command:
- 24fps
- ~500kbps bitrate
- Uses all images called frame{00000..99999}.png
- starts fading out at at frame 155 with a fade duration of 45 frames
- outputs to output.mp4
ffmpeg also gives you a ton of options about handling audio, codecs, containers, etc that Imagemagick wont, but I'm leaving that well alone. Have a play.
Edit: I was originally making this up on the spot but I can confirm that this works just as expected/described.