Successor of his uncle Danilo I, Nicholas was at first Prince, and latterly Montenegro's first and only (so far) King.
During his reign the territory of Montenegro expanded, including gaining considerable seaboard on the Adriatic, independence from Turkey was achieved and the establishment of a constitutional state with democratic rights for citizens enshrined.
The above brief summary from the Wikipedia page may be counter-balanced by the view from Rebecca West in her epic travelogue/history Black Lamb and Grey Falcon. She asserts Nicholas I was a rogue who only had an eye for self-enrichment and self-aggrandisement, and any benefit to the people and state of Montenegro was entirely subservient to these aims.
Whatever the truth, it is certainly the case that Nicholas' life and reign did not end happily. As Austro-Hungarian forces entered Yugoslavia during the Great War (1914-1918), he fled and took refuge in France, where he died in 1920.
Interestingly, his male-line descendants still live in France and are prominent professional individuals with the surname Petrovic-Njegos. Nicholas of that name uses the title Prince of Montenegro.