A while back, fantasy author Brandon Sanderson told his fans that there are 'hidden secrets' in the map of Roshar that accompanies his Stormlight Archive novels (so far, The Way of Kings and Words of Radiance). In April this year, fans at the Seventeenth Shard forum cracked one of the secrets.
Roshar is not an arbitrary artistic doodle, but is based on a Julia set, a Mandelbrot-related fractal shape. In fact, the shape of Roshar appears in the demonstration video on the related Wikipedia page and has been there since 2006, suggesting Sanderson may have simply borrowed it from that location.
This isn't the first time that a fantasy author has taken inspiration for their fantasy maps from real-world sources. Many authors tweak real maps of Europe or North America, whilst others take inspiration from nature. One fantasy novel from the 1990s, whose title I now mercifully forget, even used male genitalia as the inspiration for its landmass. There are also various mapping programmes which also use fractals to generate terrain (such as the Campaign Cartographer software family). This is the first time I've encountered a well-known fantasy author using them to generate his world, however.
Arthur C. Clarke also used Mandelbrot sets as a major theme of his 1990 novel The Ghost from the Grand Banks (arguably to the detriment of the core story, about the raising of the Titanic).