Complex sentence: In 2 Nephi 23:21, the Book of Mormon quotes Isaiah 13:21 as mentioning a "satyr", a creature from Greek mythology, which is half-man, half-goat.
Split into simpler sentences: In 2 Nephi 23:21, the Book of Mormon quotes Isaiah 13:21, which mentions a "satyr". Satyrs are creatures from Greek mythology, which are half-man, half-goat.