Megami Tensei (デジタル・デビル物語 女神転生, Dejitaru Debiru Monogatari: Megami Tensei) is the first game in the Megami Tensei series. It was developed by "Wolf Team" (the subsidiary of Nihon Telenet which is now "Namco Tales Studio"), and was released for the Famicom by Namcot in 1987 (there was also a prior version released for the MSX.
A large part of the game is devoted to scrolling, three-dimensional dungeon, something which was quite revolutionary for it's time. The game pioneered many of the features which are now trademarks of the Megami Tensei franchise. It, along with Square's Final Fantasy and Enix's Dragon Warrior, set the standard for console RPGs, and each game managed to spawn a series which would take these different formulas to new heights.
The game also focuses very heavily on demons, which have gone on to become a trademark of the series. It was later followed by a sequel, and then by the follow-up Shin Megami Tensei, (a name that would be used by most future games in the franchise). There have also been many spin-off series, including the Last Bible trilogy, Majin Tensei series, the Persona games, and the Avatar Tuner dualogy.
Megami Tensei is based on the Digital Devil Monogatari trilogy of books written by Aya Nishitani, but is not a direct adaption. It actually takes place after both Digital Devil Story: Megami Tensei and Digital Devil Story: Warrior of the Demon City, involving a scenario in which Loki and Set are resurrected by Lucifer.