Imam Hasan Askari (a.s.), the Son of the 10th Imam,was born in 232 A.H, and martyred by Mo'tamed, the Abasid caliph (according to some Shi'ite narrations).
Imam Hasan Askari (a.s.) was selected as Muslims’ Imam after the death of his honorable father.
The period of his Imamate was about 7 years. For the pressure of Abbaside caliphs, Imam tried not to have any relation with people, except some of his friends.
The reasons of this severe pressure were: first, at that time the number and the power of Shia groups were increased.
Second, the acceptance of Imamte by Shi'ite was clear for everyone. Third, Shi’ite Imams would be recognized, and would be killed by Abbasid Caliphs .
Caliphs knew that the 11th Imam had a son (according to some narrations) who was Mahdi (May God hasten his glad advent).
Therefore Abbasid Caliphs controlled the 11th Imam, more than other Imams. They decided to put an end to Imamate of the Shi'a and close it forever.
So, when Mo’tamed was informed that the 11th Imam was sick, he sent a doctor to him and also sent some of his close friends and judges to visit him and always controlled Imam to be aware of what was happening in his home.
After the martyrdom of the Imam, the caliph’s officers tried to find his son but they became hopeless. After his death, Imam was burried in his house besides his honorable father’s grave in Samarra City.