Complex sentence: Frodo, Sam and Gollum traverse the Dead Marshes, evading a Nazgûl along the way, but upon reaching the Black Gate, they find it closed and guarded by Orcs.
Split into simpler sentences: Frodo, Sam and Gollum traverse the Dead Marshes, evading a Nazgûl. Upon reaching the Black Gate, they find it closed and guarded by Orcs.