Guru Har Krishan (23 July 1656 – 30 March 1664) was the eighth of the ten Sikh Gurus. At the age of 5, he became the youngest Guru in Sikhism on 7 October 1661, succeeding his father, Guru Har Rai.
He is also known as Bal Guru (Child Guru).He died at age eight during a epidemic in Delhi, and his granduncle Guru Tegh Bahadur became the next Guru of the Sikhs.He had the shortest tenure as Guru, lasting only 2 years, 5 months and 24 days.
Har Krishan was born in Kiratpur Sahib, Rupnagar, Punjab, India to Guru Har Rai and Kishan dei (Mata Sulakhni).
Before His death in October 1661, Guru Har Rai designated his younger son Har Krishan as the next Guru. Guru Har Rai chose Har Krishan, rather than his elder son Ram Rai, because Ram Rai was in collusion with the Mughal Empire. Har Krishan was only five years old when he succeeded his father as Guru.
One of the historic gurdwaras in India, the Bangla Sahib in Delhi, was built on the site where Guru Har Krishan helped the sick. Guru Har Krishan died at Gurudwara Bangla Sahib, Delhi.
(Ref : https://en.wikipedia.org/wiki/Guru_Har_Krishan & sikhism-spreaded-over-world.blogspot.com)