Complex sentence: Shishupala or Sisupala was son of Damaghosha, king of Chedi, by Srutadeva, sister of Vasudeva; he was therefore not only cousin of Krishna, but also was Krishna's implacable foe, because Krishna had carried off Rukmini, his intended wife.
Split into simpler sentences: Shishupala or Sisupala was son of Damaghosha, king of Chedi, by Srutadeva, sister of Vasudeva. Therefore he was not only cousin of Krishna, but also Krishna's implacable foe, because Krishna had carried off Rukmini, his intended wife.