The modern history of Sikkim begins from 1642 A.D. with the coronation of Phuntsog Namgyal as the first Chogyal or king of Sikkim in a tranquil pine covered hill in Yuksom Norbugang in West Sikkim.

The Namgyals were scions of the Mi-nyak House in Kham in Eastern Tibet. It is said that there were three brothers, chiefs of Kham Mi-nyak. A letter dropped from heaven directed the middle brother to go south towards Sikkim where his descendents were fated to rule. It was in Sakya that his eldest son single-handedly raised the pillars of the Sakya monastery and earned himself the sobriquet of 'Khye Bumsa'( the strength of a lakh of men)

Khye Bumsa also earned himself the hand of the daughter of the Sakya hierarch and settled in Chumbi Valley, which remained, for a long time, the epicenter of the later kingdom of Sikkim.

Long troubled by the fact that he and his wife were issueless, Khye Bumsa sought the blessings of the Lepcha chieftain Thekongtek who was reputed to be able to grant the boon of progeny. Khye Bumsa's wife subsequently bore him three sons. Later Khye Bumsa and Thekong Tek swore the historic pact of eternal friendship at Kabi Longtsok in North Sikkim.

Khye Bumsa's third son Mipon Rab succeeded his father. He, in turn, was succeeded by his fourth son Guru Tashi who moved to Gangtok. Meanwhile Thekongtek passed away and the Lepchas who started fragmenting into small tribes turned to Guru Tashi for leadership and protection.

The Sikkim Coronation book describes Guru Tashi as the 'first ruler of Sikkim who paved way for a regular monarchy'.

Five generations later, it was Phuntsog Namgyal who was consecrated as the first Denjong Gyalpo or the king of Sikkim by the three great Lamas who came from the North, West and South to Yuksom Norbugang in West Sikkim in 1642 A.D. The event, predicted as it was by Guru Rinpoche, was the 'Naljor Chezhi' or the meeting of the four yogic brothers or the four saints or four sages.

It was preordained that three saints of great repute from different parts of Tibet make their way to Bayul Demajong (Sikkim) to discharge their responsibility of upholding and propogating the essence of Dharma in the hidden land of Demajong. Thus it was that Lhatsun Namkha Jigme, Kathog Kuntu Zangpo and Gnadak Sempa Phuntsog Rigzin made their way to Sikkim separately, and through impenetrable routes.
This historical congregation of the three holy Lamas is called Yuksom, which in Lepcha means the 'Three Superior Ones'.

Lhatsun Chenpo impressed on the other two that they were all Lamas and needed a layman to rule the kingdom righteously. He further pointed out that, 'In the prophecy of Guru Rinpoche, it is written that four noble brothers shall meet in Demajong and arrange for its government. We were three of those who came from the North, West and South'. As for the East, he quoted the oracular guide book Rinchen Lingpa which mentioned, 'One of my four avatars will be like a lion, the king among beasts, who will protect the kingdom by his bravery and powers'. The book also mentioned that, 'One named Phuntsog from the direction of Gang will appear'.

So Lhatsun Chenpo deputed a hermit called Togden Kalzang Thondup and a layman called Passang to lead a party to Gangtok in invite the person bearing the name of Phuntsog to come to Yuksom Norbugang. After several adventures, the party came to Gangtok where they met Phuntsog milking his cows. Phuntsog invited them in and bade them partake of fresh cow's milk and told them his name was Phuntsog. He saw the invitation of the three Lamas as a most fortuitous event and lost no time in setting out for Yuksom Norbugang with his entire retinue of followers, officers and household establishment.

The coronation took place in the Chu-ta or water horse year in 1642A.D. Thus Phuntsog Namgyal was installed on the throne of Sikkim with the title of 'Chogyal' or king who rules with righteousness, with both spiritual and temporal powers. While the three Lamas spread Buddhism in Sikkim, Phuntsog Namgyal started consolidating his kingdom.

Twelve generations of Chogyals ruled over Sikkim for over 300 years. This tiny Himalayan kingdom however witnessed tumultuous change in 1972-73. In 1975 the institution of the Chogyal was abolished and on May 16th, 1975 Sikkim was formally inducted as the 22nd state of India.
(content from sikkim.gov.in/ASP/Miscc/aboutsikkim.htm)