Complex sentence: The Horus Heresy was introduced to the setting soon after; it is a short but all-encompassing conflict, a civil war named for the initiating faction's leader, that devastates and redefines the, the universe's nascent galactic empire.
Split into simpler sentences: The Horus Heresy was introduced to the setting soon after as a short but all-encompassing conflict, a civil war named after the leader of the initiating faction. The war devastates and redefines the, the universe's nascent galactic empire.