The great warrior and companion Hazrat Hamza (Ra) was an uncle of the Prophet Muhammad(pbuh). His conversion to Islam was a sudden incident.
After being bestowed with Prophethood, the Prophet Muhammad(pbuh) started preaching Islam publicly. For this reason the infidels started opposition and oppression. In the meantime one day the Prophet Muhammad(pbuh) was in meditation at the foot of Safa mountain. The wicked Abu Jahl rebuked him in an extremely, objectionable language and struck him. Uttering not a word the Prophet Muhammad(pbuh) returned home. Hamza (Ra) had just returned from hunting. He came to know about this incdent. He rushed angrily in that condition to the compound of Kaba. Seeing Abu Jahl sitting among other leaders of Quraish he roared like a lion. Striking Abu Jahl's head with his bow again and again, he said, what you dare rebuke my nephew and strike him. Hamza declared so listen I recite la illaha illallahu Muhammad Rasulallah and I accept islam, the religion of Muhammad(pbuh). Hamza went to the Prophet Muhammad(pbuh) and narrated the whole incident. He also informed him about his conversion to Islam. The Prophet Muhammad(pbuh) was very pleased by acceptance of Islam by a warrior like Hamza.
Hazrat hamza (Ra) was a mighty brave warrior. He was honoured by the title of the lion of Allah his Prophet's for his incomparable bravery. After accepting islam hamza (Ra) dedicated himself to the cause of the Prophet Muhammad(pbuh) and Islam. He remained like a shadow with the Prophet Muhammad(pbuh) during different crises. When the Prophet Muhammad(pbuh) was engaged in preaching Islam security of the Prophet Muhammad(pbuh) and the Muslims rested with Hamza (Ra). Hazrat Hamza (Ra) earned name and fame for his excellent bravery in the battle of Badr. In that battle the Muslims killed many Quraish leaders including Abu Jahl.
In the battle of Uhud, Hamza (Ra) was one of the targets of the infidels for killing, because many Quraish heroes were killed by Hamza (Ra) in the battle of Badr. As a result at one stage of the battle, Abyssinian slave wahshi Ibn Harb with a spear attacked him. He fell down on the ground and was martyred.
May Allah(Swt) be pleased with him.