Complex sentence: Among the most insidious of mankind's enemies are Genestealers, a strange and terrifying alien race, reptilian in appearance, they are huge armoured six-limbed beasts, preternaturally swift and unbelievably ferocious in combat.
Split into simpler sentences: Among the most insidious of mankind's enemies are Genestealers, a strange and terrifying alien race. They are huge armored six-limbed beasts, preternaturally swift and unbelievably ferocious in combat.