Complex sentence: The book revealed that the spell had marked Sandrino with the cursed ink on his arm, with the source of his invincibility coming from his hardened heart, consumed by anger and devoid of humanity.
Split into simpler sentences: The book revealed that the spell marked Sandrino with the cursed ink on his arm. The spell that made Sandrino invincible was due to his hardened heart, consumed by anger and devoid of humanity.