Complex sentence: As described in "A Dance with Dragons", the Doom was of apparent volcanic and seismic nature taking effect for five hundred miles around Valyria, fragmenting the land surrounding the city into numerous smaller islands and thus creating the Smoking Sea between them.
Split into simpler sentences: As described in "A Dance with Dragons", the Doom was of apparent volcanic and seismic nature taking effect for five hundred miles around Valyria. The land surrounding the city was fragmented into numerous smaller islands, creating the Smoking Sea between them.