Complex sentence: In 386, Emperor Theodosius I began erecting a much larger and more beautiful basilica with a nave and four aisles with a transept; the work including the mosaics was not completed until Leo I's pontificate (440--461).
Split into simpler sentences: In 386, Emperor Theodosius I began erecting a much larger and more beautiful basilica with a nave and four aisles with a transept. The work, including the mosaics, was not completed until Leo I's pontificate (440--461).