King Palden Namgyal was the last sovereign king of Sikkim. He ruled from 1963 until 1975, when Sikkim was annexed by India and became a state of the Indian Union.

King Palden Namgyal belonged to the Namgyal dynasty, which had ruled over Sikkim for several centuries. He ascended the throne in 1963 after the death of his father, Tashi Namgyal.

During his reign, King Palden Namgyal faced various challenges and tensions, both internally and externally.

Internal Challenges:
1. Political Unrest: There were growing political tensions within Sikkim, with demands for democratic reforms and more representation for the Nepali-speaking majority.
2. Anti-Monarchy Sentiments: Some sections of the population, particularly the left-wing political groups, were critical of the monarchy and called for its abolition.

External Challenges:
1. Indian Influence: India had significant influence over Sikkim's affairs, with a military presence and control over defense and foreign policy. This influence increased over time and eventually led to Sikkim's annexation by India.
2. China's Role: China also played a role in the political dynamics of Sikkim. It had previously recognized Sikkim as an independent kingdom but later questioned its status and raised territorial claims.

Amidst these challenges, the monarchy in Sikkim faced increasing pressure. In 1975, a referendum was held in Sikkim, in which the majority of the people voted in favor of merging with India. As a result, Sikkim became the 22nd state of the Indian Union.

After the merger, King Palden Namgyal lost his sovereign status and became a private citizen. He lived in Gangtok, the capital of Sikkim, until his death in 1982.

In conclusion, King Palden Namgyal was the last sovereign king of Sikkim who ruled from 1963 to 1975. His reign was marked by political unrest, demands for democratic reforms, and increasing external influence from India and China. The annexation of Sikkim by India in 1975 led to the end of the monarchy, and King Palden Namgyal lived as a private citizen until his death.