Complex sentence: During the fight, Seiya is mortally wounded when he protects Athena from the sword of Hades and his Cosmo disappears, thus leading readers to believe the Pegasus Saint died.
Split into simpler sentences: During the fight, Seiya is mortally wounded when he protects Athena from the sword of Hades and his Cosmo disappears. Thus leading readers to believe the Pegasus Saint died.