Complex sentence: The race called Chitauri appeared as Ultimate Marvel's (Earth-1610's) counterpart to the Skrulls, the Chitauri are a shapeshifting alien species who have attempted to conquer the Earth, most notably during World War II and again in the early 21st century.
Split into simpler sentences: The race called Chitauri appeared as Ultimate Marvel's (Earth-1610's) counterpart to the Skrulls. They are a shapeshifting alien species who have attempted to conquer the Earth, most notably during World War II and again in the early 21st century.