Complex sentence: The great dwarven smith, Durgeddin the Black, founded the secret stronghold within a great underground cavern system two hundred years ago when he and his clan were driven from their home by a horde of orcs and trolls.
Split into simpler sentences: The great dwarven smith, Durgeddin the Black, founded the secret stronghold within a great underground cavern system two hundred years ago. When he and his clan were driven from their home by a horde of orcs and trolls.