Complex sentence: He's actually Alicia's son but believes she abandoned him because of his element power, although he witnessed her being taken away by the Alteans, and in that moment, he was split into two different beings by Mykage, the other being Kagura.
Split into simpler sentences: He's actually Alicia's son, but believes she abandoned him because of his element power. Even though he witnessed her being taken away by the Alteans, it was during that moment, that he was split into two different beings by Mykage, the other being Kagura.