Final Fantasy VI is a game where the villain achieves their goal and manages to acquire godlike powers, before laying waste to the world. Kefka the mad clown almost succeeds in destroying Terra and her allies, but the party manages to reform and they ascend into the heavens in order to destroy Kefka once and for all.
The second half of Final Fantasy VI takes place in the World of Ruin, which is a devastated landscape that has been created by Kefka's powers. The player is given four characters and can either choose to search the world and find the rest of their allies or they can go straight to the end boss battle.
It turns out that Final Fantasy VI was almost a very different game, as the entire concept of the World of Ruin was a late addition to the story. There is an interview with many of the developers of Final Fantasy VI in an upcoming issue of Famitsu that has been translated into English by BClarkOMP of One Million Power, where it is revealed that Final Fantasy VI almost ended a lot sooner.
(Hironobu Sakaguchi) "...It was a shocking development when the world was destroyed, and you continued traveling through the World of Ruin afterward.
Actually, we didn’t initially plan to make the World of Ruin at all."
(Yoshinori Kitase) "We originally planned for the party to save the world and defeat Kefka just as things were looking grim and the world was about to be destroyed. Then we started talking about reworking that."
It turns out that Final Fantasy VI was going to have a much more standard JRPG ending, with the party defeating Kefka before he could succeed in his plan. It's for the best that the revisions to the ending were made, as the World of Ruin and the open-ended nature of its quests are one of the most memorable aspects of Final Fantasy VI.
Final Fantasy VI has one of the darkest tones in the series and the World of Ruin exemplifies its themes better than the rest of the game. A version of Final Fantasy VI that ends with Kefka being defeated before achieving godlike power would have been a far inferior product than the one we were lucky enough to receive.