Complex sentence: The game was written, designed, and composed for by creator Toby Fox, with additional artwork by Temmie Chang, and was released for Microsoft Windows and OS X on 15 September 2015.
Split into simpler sentences: The game was written, designed, and composed by its creator Toby Fox, with additional artwork by Temmie Chang. It was released for Microsoft Windows and OS X on 15 September 2015.