Complex sentence: Two years have passed since the vampires, led by the Master, used atomic weapons to create a nuclear winter, blocking the sun almost continuously and allowing the vampires to move freely, except for 2--3 hours when sunlight filters through.
Split into simpler sentences: Two years have passed since the vampires, led by the Master, used atomic weapons to create a nuclear winter. This successfully blocked out the sun and allowed the vampires to move freely, except for two to three hours when sunlight filters through.