Complex sentence: Ali ibn Husayn was the only son of Hussein ibn Ali who survived the Battle of Karbala in 680 when he was twenty three years old, for he could not take part in the battle due to his illness and was thus saved from the general massacre.
Split into simpler sentences: Ali ibn Husayn was the only son of Hussein ibn Ali who survived the Battle of Karbala in 680 when he was twenty three years old. For, he could not take part in the battle due to his illness, and was thus saved from the general massacre.