外传 has two pronunciations: wàizhuàn (unofficial biography / spin-off story) and wàichuán (to divulge a secret). The spin-off meaning is far more common in modern usage, especially for games and media.
grammar
As wàichuán (verb): 这件事不能外传 (This matter must not be leaked). As wàizhuàn (noun): 仙剑奇侠传外传 (Legend of Sword and Fairy spin-off).
