Short answers, pulled from the story.
Nicholas I reigned as Emperor of Russia, King of Poland, and Grand Duke of Finland from 1825 to 1855, a period of thirty years. His reign began on the 14th of December 1825 following the death of his brother Alexander I and ended with his own death on the 2nd of March 1855.
The Decembrist revolt took place on the 26th of December 1825, the day after Nicholas I issued his accession manifesto. Around 3,000 young army officers and liberal citizens gathered to demand a constitution and representative government. Nicholas ordered the army to suppress it, and the trauma of facing a revolt on his first day in power drove him to build a massive surveillance state under Alexander Benckendorff, employing 300 gendarmes and a network of informers.
Nicholas I introduced a military conscription edict on the 26th of August 1827 requiring Jewish boys to serve 25 years in the Russian military from age 18, with many forcibly sent to Cantonist schools from age 12. Between 1827 and 1854, an estimated 70,000 Jews were conscripted. Nicholas also abolished the Jewish Qahal in 1843 and restricted Jewish book printing to only two cities, Zhitomir and Vilna.
Nicholas I lost the Crimean War due to a combination of structural military failures, administrative incompetence, and diplomatic miscalculation. The Russian army was trained for parades rather than combat, colonels sold their men's best equipment and food, and no railroad had been built toward the Ottoman frontier. By 1854, Russia faced Britain, France, the Kingdom of Sardinia, and the Ottomans together, with Austria and Prussia neutralized diplomatically.
In 1833, Sergey Uvarov of the Ministry of National Education devised the program of "Orthodoxy, Autocracy, and Nationality" as the guiding principle of Nicholas I's regime. It demanded loyalty to the unrestricted authority of the tsar, adherence to the Russian Orthodox Church, and the primacy of the Russian language. The policy led to the persecution of non-Russian languages and non-Orthodox religions across the empire.
Nicholas I received Hector Berlioz with particular favor, sending him a lavish diamond-encrusted ring before Berlioz's concert tours in Russia in 1847, and Berlioz later dedicated his Symphonie fantastique to the emperor. His relationship with Franz Liszt was more strained; accounts describe an incident at the Winter Palace where Liszt reportedly halted a performance in response to the tsar talking during the music. Liszt nonetheless maintained connections to the imperial family through Grand Duchess Maria Pavlovna, the tsar's sister.