Complex sentence: Immigrants to the United States brought with themdwarves, elves, leprechauns, and other spirits and gods, but power of these mythological beings has diminished as people's beliefs wane.
Split into simpler sentences: Immigrants to the United States brought with them dwarves, elves, leprechauns, and other spirits and gods. However, the power of these mythological beings has diminished as people's beliefs wane.