Who was Ali al-Asghar

He was the youngest son of Imam Hussain and Rubab (the daughter of the chief of the Kinda Imra al-Qays tribe)
He is honoured by all muslims as being the youngest martyr (6 months old) at the battlefield of Karbala.
Abdullah "Ali al-Asghar" ("Youngest Ali") ibn Hussain was born in Medina, and was one of the three sons of Hussain. The other two were Imam Zain ul-Abideen, and Ali Akbar ibn Hussain, who Yazid's forces also killed in the Battle of Karbala. His sister was Sakinah bint Hussain.

On the morning of 10th Muharram

Ali Asgher, fell from the cradle hearing the cry of his father Hussain. Hussain Ibn-Ali was called, and when he came, he saw the thirst that was gradually killing Ali Asgher.
Therefore, he asked permission of his wife who was the mother of Ali-Asgher to take him to the battle field and ask his enemies for water.
Hussain introduced his son Ali-Asgher and asked water for his son. The Army of Yazid was so grieved that they began questioning their belief in Yazeed and his ruling.
Ibn-Zyad, the Commander of the Army of Yazid, ordered Hurmala to kill this child.

Hussain, in seeing this, put Ali-Asgher on the ground and told them that he would not take the water and that they themselves should give this six-month old child water.

Ali Asghar in the Battlefield

Hurmala was one of the greatest archers at the time, and he shot the arrow which then pierced the neck of Ali Asgher.
This arrow was a three-headed arrow. When it pierced the neck of Ali-Asgher killing him, the equilibrium of his body was unstable.
So much so that his body turned. This is the depiction of the weight of the three-headed arrow which was used for hunting deers.

As a result, Hussain Ibn-Ali was grieved and weeped. He did not know how to take this child of his back to his mother, its inform that Hussain approached the entrance to the tent of his infants mother Rubab seven times as if he was building up the courage to face Rubab who would be runied, disraught and will never again smile.
While approaching back and forth seven times Hussain recited
'Verily we are Allah’s and verily unto Him we return. We are hapy with His will and carry out His command'.

After the Martyrdom of Imam Hussain

When Hussain was beheaded and murdered, Ibn-Zyad announced that anyone who could bring the sever heads of all the companions of Hussain, would be rewarded.
Eventually, all of Hussain's companions' severe heads were brought.
Ibn Zyad questioned the head of the six-month old child Hurmala had killed. One of the people of Yazid's Army explained that he saw Hussain Ibn Ali burrying his son. Therefore, Ibn Zyad demanded that his army should go around the camp of Hussain and stabbing the ground with their swords and spears until they find the grave.

As a result, one of the soldiers found the body and beheaded Ali-Asgher's head from his body. His severe head, along with all the companions' heads was raised on spears.
The ladies of Hussain who were taken prisoners were constantly reminded of their loved ones especially the wife of Hussain and the mother of the six month old Rubab.