Complex sentence: He wished Magni a heartfelt farewell and promised he would never forget him, then returned to the past with Tarene's help and changed his younger self to ensure that the future he had experienced would never come about.
Split into simpler sentences: He wished Magni a heartfelt farewell and promised he would never forget him, then returned to the past. With Tarene's help, he warned younger self to ensure that the future he had experienced would never come about.