Final Fantasy II Muma no Meikyū

Final Fantasy II Muma no Meikyū (ファイナルファンタジーII 夢魔の迷宮, lit.  Final Fantasy II Nightmare's Labyrinth) is a novelization of the plot of Final Fantasy II written by Kenji Terada, one of the game's original writers and published by Kadokawa Shoten.  It reworks the plot of Final Fantasy II and includes additional information of certain characters, such as the Emperor of Palamecia's real name. 

The novel has some changes to the game's storyline, some small ones and some rather major.  Due to this, it can only be considered semi-canon. 

Ricard, Cid and Josef are not in the novel at all; their storylines completely cut. 

The novel starts a bit before the start of the game showing Firion, Leon, Maria and Guy's normal life.  Firion and Maria end up together in the end, rather than the ambiguous ending of the game. 

Guy's backstory is explained that he was raised by monsters, and thus is able to speak their language.  Guy's ability is more important in the novel, where Ricard's omission means Guy acts as the translator between the Wyvern and the party. 

Emperor Mateus's backstory and character is explained in the novel.  It's stated that Palamecia is cursed with demonic influences that corrupt its emperors.  The devil himself, Satan, desiring to rule the world by human proxy, turns men into malicious masterminds by using everything at his disposal to pollute their thoughts with his malice.  The devil originates all the way from the beginning of the world when he became trapped within the indestructible "Stone of Iludia" and swore revenge by any means.  After ascending to the throne, Mateus exiles his mother Airu to the deserts of Palamecia and yet she still spends her life trying to find a way to save him.  During the novel, Airu and Minwu team up to find a crystal, which can purify the land.  It ends up being too late to save Mateus, but it helps to bring peace to the world.  Mateus's father is stated to have been just as evil as him, having sold his soul to Satan to gain more power. 

There is no Pandaemonium; Mateus is killed when Firion and his friends go to rescue Hilda near the end of the story.  The novel's last chapter is about destroying the Stone of Iludia (which keeps corrupting all of the Palamecian emperors) with the crystal created by Minwu and Airu. 

The "Hell" version of Mateus is actually Satan taking the likeness of Mateus.