https://www.thetoptens.com/hardest-video-games/
http://www.denofgeek.com/us/games/video-games/249481/top-25-brutally-difficult-video-games/page/0/1
https://www.theguardian.com/technology/2016/mar/18/the-25-hardest-video-games-of-all-time
https://en.wikipedia.org/wiki/Battletoads
https://en.wikipedia.org/wiki/Battletoads_(video_game)
https://en.wikipedia.org/wiki/Dark_Souls_II
http://www.escapistmagazine.com/forums/read/9.387764-Understanding-Dark-Souls-difficulty
https://en.wikipedia.org/wiki/Shinobi_(2002_video_game)
https://en.wikipedia.org/wiki/Ninja_Gaiden_(2004_video_game)
https://en.wikipedia.org/wiki/Ghosts_%27n_Goblins_(video_game)
http://tvtropes.org/pmwiki/pmwiki.php/VideoGame/SilverSurfer
https://en.wikipedia.org/wiki/Contra_(video_game)
https://en.wikipedia.org/wiki/I_Wanna_Be_the_Guy
http://www.ign.com/boards/threads/anyone-else-think-mega-man-9-is-too-hard.171557084/page-2
https://en.wikipedia.org/wiki/Super_Meat_Boy