Aun and Muhammad were the sons of Bibi Zainab. They had not accompanied Bibi Zainab when she left Madina with Imaam Hussain (A). Just before Imaam Hussain started his journey from Mecca, Hazrat Abdullah ibne Jaffer brought his two sons to Mecca and handing them over to Imaam Hussain said, "Ya Imaam, since you have decided to go and will not allow me to come with you, please take my two sons with you. Aun will represent his maternal grandfather Hazrat Ali (A) and the other will represent his paternal grandfather Hazrat Jaffer-e-Tayyaar".
Aun and Muhammad were quite young. It is reported that Aun was about thirteen and Muhammad was a year or two younger. They had learnt the art of fencing from their uncle, Hazrat Abbas.
On the night before Ashura Bibi Zainab said to them, "My sons, tomorrow there will be a battle. I can not ask you to fight because you are young. But if anything happens to Imaam Hussain, while you are still alive, I will be filled with shame." Both the boys stood up and said "Mother, we have the blood of Ali and Jaffer in our veins. Our grand fathers were warriors whose fame will always be remembered. Do you think we can possibly shame them? More over we are the pupils of Uncle Abbas. Mother, unless you forbid us and stop us from fighting, we shall go to the battlefield and show the enemies of Islam how bravely the children of Islam can fight. All we want from you is a promise that you will never weep for us. Or souls will never rest in peace if you grieve for us after we are gone".
Tears of joy and pride flowed down Bibi Zainab's eyes as she embraced her two boys. In the morning during the general attacks from the enemy, Aun and Muhammad fought side by side with Ali Akber, Qasim and Hazrat Abbas. Every time either of them succeeded in felling an enemy, he would look proudly at Hazrat Abbas who would smile and nod his approval. Imaam Hussain would not, however, give the two boys permission to go for single combat. They were very disappointed. They came to their mother for help. Bibi Zainab sent someone to request Imaam Hussain to come to her tent., When the Imaam came Bibi Zainab said, "Hussain, at the battle of Siffeen Abbas was only eight years old. When he saw someone trying to attack you, he rushed into the battle field and killed the man. Do you remember how proud our father Ali was? Today I too want to be proud of my sons. I want to see them go out there and defend Islam. Will you not allow me that privilege?" Imaam Hussain stood there in silence. He looked at his sister. He saw the disappointment on her face. He saw tears forming around her eyes. He put his arms around the two boys and led them to their horses. He kissed them and then helped them mount. "Go," said Imaam, "Go, and show the world how those as young as you can fight the injustice and oppression of Yazid!" Then he turned round and lifted the curtain of the tent. The boys raised their hands and said "Fi Amaani-llah, mother!" Bibi Zainab replied, "Bismillah my sons. Allah be with you!"
The two boys rode out into the battlefield. They fought bravely. At one point Umar Sa'ad asked, "Who are these two youngsters? They fight like I have seen Ali ibne Abu Taalib fight." When he was told who they were he ordered his soldiers to give up single combats and surround and kill the boys. Aun and Muhammad were attacked from all sides. Soon they were over-powered and brutally killed. Imaam Hussain and Hazrat Abbas carried the two young bodies to a tent and laid them on the floor. Imaam walked to Bibi Zainab's tent. He found her in sijdah praying, "Ya Allah, I thank you for accepting my sacrifice. My heart is filled with pride because my two sons have given their lives for your religion."
Reference: "The Journey of Tears" by Marhum Mulla Bashir Rahim [http://www.al-islam.org/journey-of-tears]
Having heard al-Husain's speech and his plea for help, al-Hurr came to ‘Umar Ibn Sa’d and said, “Are you going to fight this man?” “Yes, by Allah,” said ‘Umar, adding, “a fight in the easiest part of which heads will roll down and hands will be cut off.” Al-Hurr asked him, “What is your objection to his offer of departure?” ‘Umar answered: “Had it been up to me, I would have accepted it, but your governor refuses.”
Al-Hurr left him and stood by the others. Beside him stood Qarrah Ibn Qays whom he asked, “Have you watered your horse today?” “No,” came the answer. “Do you then wish to do so?” was al-Hurr's question. Qarrah took that statement to imply that al-Hurr was reluctant to fight al-Husain (A) and did not wish to be seen by him defecting, so he walked away from him. Al-Hurr kept getting closer and closer to al-Husain (A). Al-Muhajir Ibn Aws asked him, “Do you want to charge at him?”
Al-Hurr remained silent. He felt chilled to the bones, so he shivered. Having seen him shiver, al-Muhajir felt terrified and said to him, “Had I been asked: ‘Who is the most daring of all the Kufians?', I would have given no name other than yours; so, why do I see you look like that?” Al-Hurr said, “I am giving my soul the option between choosing Paradise or hell. By Allah! I do not prefer anything over Paradise even if it means I will be burnt alive.” Having said so, he beat his horse in the direction of al-Husain (A).
Turning his spear upside down and holding his shield the opposite way, he came lowering his head, feeling too shy to look at the Prophet's family in the eyes because of having exposed them to such hardship, bringing them to such a place where neither water nor grass could be found. Loudly he spoke these words:
“O Allah! To You do I surrender, so do accept my repentance, for I have filled the hearts of Your walis and the sons of Your Prophet with fear! O father of ‘Abdullah! I am repentant; so, can my repentance be accepted at all?”
Al-Husain (A) said, “Yes. Allah will accept your repentance”. This statement found its place to al-Hurr’s heart, filling it with joy. He took a moment to contemplate upon the eternal life and the incessant bliss. It now became clear to him what that voice, which had addressed him, meant upon his departure from Kufa. He had a dialogue with al-Husain (A). Among what he said to him was:
“When I went out of Kufa, I was addressed thus: “O Hurr! You are given the glad tidings of [going to] Paradise!” I said to myself, “Woe unto me! How can I be given such glad tidings since I am going to fight the son of the daughter of Allah's Messenger?!” 
Al-Husain (A) said to him, “You have now acquired a great deal of good and a great reward.” A Turkish slave was with him. 
al-Hurr admonishes the Kufans
Al-Hurr sought al-Husain's permission to address the people, and permission was granted to him. As loudly as he could, al-Hurr called out to the Kufians thus:
“O people of Kufa! A foolish and a bad example for others have you surely set when you invited him to come to you then grieved him and surrounded him from all directions, forbidding him from going anywhere in Allah's spacious land so that he and his family might be safe, rendering him like a captive in your hands, unable to help himself. You have prohibited him, his ladies, his children, and his companions from the flowing water of the Euphrates of which the Jews, the Christians, and the Zoroastrians drink and wherein black swine and dogs wade! Look and see how thirst has subdued them! Evil is the way whereby you have succeeded Muhammad (S) in treating his progeny! May Allah never permit you to drink on the Day of Thirst!”
His own men now started shooting him with arrows, so he was forced to retreat till he stood face-to-face with Imam Husain (A). 
After the martydom of Habib, al-Hurr Ibn Yazid al-Riyai came out accompanied by Zuhayr Ibn al-Qayn who was protecting him from the rear. Whenever one of them attacked and the situation became critical, the other would attack to rescue him, and they kept doing so for a while.  The horse on which al-Hurr was riding received hits on its ears and eyebrows, and it was bleeding as its rider was quoting the following verse by Antar Ibn Shaddad al-’Abasi:
I kept shooting them at its very mouth,
At its chest, till blood drenched it all.
Al-Hasin said to Yazid Ibn Sufyan, “Is this al-Hurr whom you wished to kill?” “Yes,” said Yazid, so the first came out and challenged al-Hurr to a duel. It turned out that al-Hasin was asking for a swift death, for it did not take al-Hurr long to kill him! Ayyub Ibn Mashrah al-Khaywani shot al-Hurr's horse with an arrow, hamstringing it. The poor horse leaped, so the rider leaped from it like a lion,  holding his sword in his hand. He kept fighting on foot till he killed more than forty men. 
A company from the footmen fiercely attacked him and killed him. Al-Husain's companions carried his body and put it before the tent in front of which they were fighting. They were doing so whenever a man was killed, and al-Husain (A), each time, kept repeating this statement: “He has been killed as prophets and the offspring of prophets are killed.” 
Al-Husain (A) turned to al-Hurr, who was breathing his last, and said to him, as he wiped out the blood from his face, “You are al-Hurr [which means: the free man], just as your mother named you, and you are free in this life and in the life hereafter.” One of the companions of al-Husain (A), who some say was [al-Husain (A)’s son] ‘Ali Ibn al-Husain, eulogized him with the following verses which some people claim the Imam (A) himself had composed: 
How good al-Hurr of Banu Riya!
How patient when the lances intertwined!
How good al-Hurr when he defended Husain!
And in the morning his life he sacrificed!
To be continued...
Maqtal al-Husain: Martrydom Epic of Imam al-Husain (A) by 'Abd al Razzaq al-Muqarram
Published by: Al-Kharsan Foundation for Publications, Beirut, Lebanon, 1426 AH/2005 AD
 al-Tabari, Tarikh, Vol. 6, p. 244.
 Ibn Nama, Al-Luhuf, p. 58. as-Saduq, Al-Amali, p. 97, majlis 30. ‘Ali Ibn Muhammad al-Fattal al-Naishapuri, Rawat al-Wa’iin, p. 159.
 Ibn Nama, Muthir al-Azan, p. 31. On p. 9, Vol. 2, of his book Maqtal al-Husain, al-Khawarizmi says that he [al-Hurr] had a Turkish slave with him.
 Ibn al-Athir, Vol. 4, p. 27.
 al-Tabari, Tarikh, Vol. 6, p. 252. Al-Bidaya, Vol. 8, p. 183.
 al-Tabari, Tarikh, Vol. 6, pp. 248 and 250.
 Ibn Shahr Ashub, Manaqib, Vol. 2, p. 217 (Iranian edition).
 This text is quoted from p. 118 of Taallum al-Zahra’ of Raiyy ad-Din al-Qazwini, from p. 135, Vol. 13, of al-Nu’mani's Ghayba. According to p. 256, Vol. 6, of al-Tabari's Tarikh and p. 30, Vol. 4, of Ibn al-Athir's book, as well as al-Mufid's book Al-Irshad, a tent was placed on the battlefield, but these authors did not mention al-Husain (‘a) by name due to the magnanimity of the situation.
 ’Abdullah Nur-Allah al-Bahrani, Maqtal al-’Awalim, p. 85. al-Khawarizmi, Maqtal al-Husain, Vol. 2, p. 11.
 ’Ali Ibn Muhammad al-Fattal al-Naishapuri, Rawat al-Wa’iin, p. 160. as-Saduq, Al-Amali, p. 97, majlis 30.