Complex sentence: Dante and Virgil are placed there by the giant Antaeus, there are other Giants around the rim chained, however Antaeus is unchained as he died before the Gigantomachy.
Split into simpler sentences: Dante and Virgil are placed there by the giant Antaeus. There are other Giants around the rim that are chained; however Antaeus is unchained as he died before the Gigantomachy.