Complex sentence: Once inside, they find the bodies of many dwarfs, with a dying dwarf telling them a group of dwarves came to Gundabad to activate a secret weapon with which to destroy the orc army.
Split into simpler sentences: Once inside, they find the bodies of many dwarfs. A dying dwarf tells them a group of dwarves came to Gundabad to activate a secret weapon to destroy the orc army.