Complex sentence: In the very last scene Veronica Cartwright's character is happy to see the hero, played by Donald Sutherland, only for him to emit the alien scream, demonstrating that despite their best efforts they were unable to stop the alien force.
Split into simpler sentences: In the very last scene Veronica Cartwright's character is happy to see the hero, played by Donald Sutherland, only for him to emit the alien scream. Showing that despite their best efforts they were unable to stop the alien force.