Short answers, pulled from the story.
The main plot of Final Fantasy IV: The After Years involves the reappearance of the second moon seventeen years after the original game, which threatens to destroy the Blue Planet. The story follows the original heroes and their children as they confront a complex web of relationships involving the Hooded Man, the Man in Black, and the dark halves of the heroes themselves.
The true villain in Final Fantasy IV: The After Years is an entity known as The Creator, who created the Crystals to monitor the progress of life on each planet. The Creator decided that if a world did not evolve to its fullest potential, it must be destroyed, and the second moon was designed as a weapon to wipe out the Blue Planet.
Final Fantasy IV: The After Years was originally released to the Japanese mobile phone market in 2007. The game reached a benchmark of one million downloads in the first five months following its initial release, and by December 2010, it had exceeded 4.5 million paid downloads.
Final Fantasy IV: The After Years consists of eight supplemental tales released in intervals of about four weeks, followed by a semifinal installment and a finale released in two parts. The complete game was bundled with Final Fantasy IV and a new scenario Final Fantasy IV: Interlude as the PlayStation Portable compilation Final Fantasy IV: The Complete Collection, released in 2011.
Final Fantasy IV: The After Years was developed by Matrix Software with Takashi Tokita as producer. The game was in development in 2007, and the choice to keep the 2D sprites was to allow as many chapters to be made as possible while also invoking the feeling of nostalgia for its predecessor.