Complex sentence: The planet Arrakis itself is completely covered in a hostile desert ecosystem, however it is sparsely populated by a human population of seemingly-native Fremen, ferocious fighters who have adapted to the harsh climate and secretly can ride the planet's giant sandworms.
Split into simpler sentences: The planet Arrakis itself is completely covered in a hostile desert ecosystem. It, however, is also sparsely populated by a human population of seemingly-native Fremen, ferocious fighters who have adapted to the harsh climate and secretly can ride the planet's giant sandworms.