Sikkim's last sovereign king was Palden Namgyal.

Background:
Sikkim was an independent kingdom until 1975 when it was merged with India as its 22nd state. The Namgyal dynasty ruled Sikkim for almost 300 years until its merger with India. Palden Namgyal was the last king of Sikkim.

Life of Palden Namgyal:
Palden Namgyal was born in 1923 in Gangtok. He was the son of the 11th Chogyal (king) Tashi Namgyal. He ascended to the throne in 1965 after the death of his father. Palden Namgyal was a progressive king and was known for his modern outlook. During his reign, he made efforts to modernize Sikkim and improve the standard of living of its people.

Merger with India:
In 1975, a referendum was held in Sikkim to decide whether it should remain an independent kingdom or merge with India. The referendum resulted in a majority in favor of the merger. On May 16, 1975, Sikkim became the 22nd state of India and Palden Namgyal lost his title as king.

Later Life and Death:
After the merger, Palden Namgyal remained in Sikkim and was granted a privy purse by the Indian government. He continued to be active in social and cultural activities in Sikkim. He died in 1982 at the age of 59.

Conclusion:
Palden Namgyal was the last king of Sikkim who ruled the kingdom during a crucial period in its history. He was known for his modern outlook and his efforts to modernize Sikkim. His reign came to an end with the merger of Sikkim with India in 1975.