Sri Guru Har Krishan Sahib Ji (Gurmukhi: ਗੁਰੂ ਹਰਿ ਕ੍ਰਿਸ਼ਨ ਸਾਹਿਬ ਜੀ) (Wednesday, 23 July 1656 – Saturday, 16 April 1664) was the eighth Guru Nanak Dev ji (divine messengers) of Sikhism. He was the second son of the seventh Guru, Sri Guru Har Rai Sahib Ji and Mata Krishan Kaur Ji (Sulakhni Ji). Before passing away in 1661, Guru Har Rai Sahib Ji had declared that his younger son Har Krishan, would be the next Sikh Guru. Thus, Har Krishan Sahib ascended to the throne of Guruship on Sunday, 20 October 1661, at the young age of five years.
In human history, there are but few devotees of God who have achieved a supreme level of spirituality in such a tender age, as Guru Har Krishan Sahib did. Despite his young age, Guru Sahib used to delight the hearts of his disciples by his commentaries on the passages from the holy scripture, the Guru Granth Sahib. He reminded people to cherish the One God alone, asking them to discard passions and learn the virtues of patience, charity and love.
During the year 1663, when Guru Har Krishan Sahib was in Delhi, a swear epidemic of cholera and smallpox broke out. The seven year old Guru attended to and served the suffering people with complete devotion. By his grace, the lake at Bangla Sahib provided a cure for thousands of peoples. In the process of serving the diseased, the Guru was himself seized by high fever and an attack of smallpox, eventually passing away on 30th March 1664, aged just eight years. The Guru wished that nobody mourn his death, rather instructing the Sikhs to sing hymns from the Gurbani. The Tenth Nanak, Guru Gobind Singh Sahib, later paid tribute to Guru Har Krishan Sahib by stating “Let us think of the holy Har Krishan, whose sight dispels all sorrow”, which forms a part of the daily Sikh prayer Ardas.
Before passing away, when Guru Har Krishan was questioned on his successor, he merely exclaimed “Baba Bakala ji”, thus guiding Sikhs to the village Bakala near river Beas in the Punjab province, the location of the next Guru, Sri Guru Teg Bahadur Sahib Ji.
More details: https://www.sikhiwiki.org/index.php/Guru_Har_Krishan