Originally a creepypasta, it has developed into an urban legend due to the emergance of "evidence" of the game's existance. It supposedly is a demonic game that is hidden within other games.
Misfortune.gb is a gameboy puzzle game that plays disturbing music. It is considered the scariest video game ever created. According to both misfortune-dot-gb.webs.com and the creepypasta wiki,
"The game revolves around what appears to be a little boy who meets a malevolent being in a strange Gothic building. The being never gives its name, but it is heavily implied that it is The Devil. Upon meeting the creature, a dialog box appears with the text: 'I exist within the very fabric of reality. Do you want to challenge me?' This is followed by a yes or no choice. Should the player choose yes, the being replies 'Then, let's begin.'
The player is then transported to a series of maze like rooms, each filled with pit drops, locked doors, keys and traps. The objective for the player is to survive each room by either reaching the stairs to the next level, or solving another kind of puzzle. This can be a riddle or something else, such as picking a correct door. A good example is the level where four small cabins are shown on screen and the dialog box appears that reads 'Choose wrong and misfortune will befall your loved ones. Are you ready to play?' Should the player make a mistake or a choose a wrong exit etc, the screen will cut to black for a second before showing a screen of a higher resolution of the demon with a dialog box underneath that reads 'I am God here.' with blood styled writing, possibly being the inspiration for Creepypasta's such as the well known Sonic.exe
Some people who played this game and lost suddenly began to suffer from depression and there were several cases in which people committed suicide after losing the game. Other people became jittery in their everyday life, and some became physically sick. However, if they were able to complete the challenges they seemed to continue their everyday life with no change. Also, if a player was to make a mistake in game that would lead to game over, but the player quickly turned the Game Boy off, they would suffer no repercussions. It appeared to be viewing the game over screen that was responsible. This lead the few people who know of the game to wonder why exactly, losing the game would cause such drastic effects. The answer was the music.
The music in this game is generally mature, and although being limited to the 8 bit Game Boy sound bank, the music was very dark and disturbing at best. However, the "I am God here" game over screen had especially disturbing, dis-harmonic music that accompanied it and it is believed that this was the cause of "Misfortune" to the player. It consisted of deep buzzy tones and off key melody and the general sound of the music was damaging to brain wave patterns."
The game is not on it's own cartridge, but hidden in other game-boy titles, meaning you may already have the game. The game is generally accessed through glitches.
For the full description, check out the Creepypasta wiki article, or the link in the following section.
A website dedicated to the game was created in hopes of gathering more information about the game. Screenshots and music from the game have been posted on various websites, including Youtube.
A gallery of the cutscenes that supposedly ame from the game