What Are ROM Hacks?
In the context of ROM hacks, a ROM is a flash of a console game, saved in digital format. A ROM hack takes data from the game and changes it. Hackers may change individual lines of dialogue, game values such as hit points and attack power, and even the images used for the characters and terrain. Some hacks translate the dialogue of the game into another language, producing an unendorsed localization. Hacks can be released as "patches" which contain no game content, or "hacked ROMs" which are essentially ROMs with the patch applied.
Rom Hacks and the Law
ROM hacks exist in a legal grey area. Though the creation of a ROM may be illegal for games, games without copy protection such as SNES cartridges can be legally copied, if not distributed. Hacks themselves potentially fall under the doctrine of "fair use," depending on the nature of the work and the derivative. No lawsuit involving ROM hacks has ever gone to court, though precedent may be found in "Lewis Galoob Toys, Inc. v. Nintendo of America, Inc." which found that hacking a game to change its difficulty is not against copyright. On the other hand, game companies can and do send cease-and-desist letters against hackers.
"Scarlet Echoes" and "Flames of Eternity"
Two famous Chrono Trigger ROM hacks are "Scarlet Echoes" and "Flames of Eternity," the latter of which was based on the former. Both hacks significantly changed the original game, adding a new storyline, new sprites, and new regions. Days before "Scarlet Echoes" was scheduled to be released, SquareEnix sent a cease-and-desist order to its creators, and the project was canceled. Nonetheless, versions appeared on Reddit and Blogspot the following year.
Other Chrono Trigger ROM Hacks
"Prophet's Guile" is a smaller hack of "Chrono Trigger" by the team behind Crimson Echoes. It tells the story of Magus when he arrived in his home time of 12,000 BC and disguised himself as the Prophet. "Chrono Trigger Retranslation" is an alternative version of the game from the original Japanese. "Chrono Trigger Hard Mode" is a version of "Chrono Trigger" with increased difficulty.