Complex sentence: Apocalypse has also been referred to as an External on several occasions; his longevity is believed by Selene to be mostly due to technological means, although Apocalypse was already many centuries old before his first encounter with the technology he would later integrate himself with.
Split into simpler sentences: Apocalypse has also been referred to as an External on several occasions, however, his longevity is believed by Selene as mostly due to technological means. Although, Apocalypse was already many centuries old before his first encounter with the technology he would later integrate himself with.