Imam Hasan al-Askari (December 6, 846 - January 4, 874), was the eleventh Shia Imam. His given name was Hasan ibn Ali ibn Muhammad.
Hasan al Askari, whose ancestor was the prophet Muhammad was born in Samarra to Ali al-Hadi and Saleel. Hasan would live almost his entire life under house arrest in Samarra. His title al-Askari derives from the Arabic word asker which means Army. Hasan's title was reflective of his living most of his life in a garrison town.
He was very knowledgeable and despite being confined to house arrest for almost his entire life, Hasan al-Askari was able to teach others about Islam, and even compiled a commentary on the Quran that would be used by later scholars. Despite his confinement as a prisoner, he was occasionally allowed to go to Baghdad, although it was under guard, and he was able to marry a Byzantine slave woman who was a granddaughter of a Roman emperor. Her name was Nargis.
Hasan died at the young age of 27 on January 4, 874, by poison. He left only one son, Muhammad al-Mahdi who was five at the time of Hasan's death (although many Sunni muslims deny his existence). Hasan's funeral was attended by many people, including the Abbasid caliph Al-Mu'tamid who had actually killed him. Hasan's brother Jafar was getting ready to lead the funeral prayer when, Hasan's son came out and told his uncle “set aside uncle, only an Imam can lead the funeral prayer of an Imam”  (http://www.al-islam.org/kaaba14/14.htm). Upon which Jafar stepped aside, and the prayer was led by the five-year old son. His son disappeared, eluding the Abbasids who were sent to find him. The Shia believe he will return at the end of time and will bring astonishing levels of science and justice for the world.
Hasan al-Askari is buried next to his father in Sammara, their mausoleum is a very important site for muslems, specially the Shia.