The Dink Network

There is also Shadowlord from the game Nier. If you know the storyline behind the game this song is both epic and saddening. A remix of the song was made by someone on YT called ShiroiYuriNoOtomi. Better than the original in my opinion and a hell of a lot better than the Whitenote Remix (Which came in the official DLC).
(Okay, I'm breaking my own rules by adding this as well since this isn't boss fight music, but this is from the same game and is the saddest theme ever. Emil- Sacrifice