Complex sentence: Alexandra Feodorovna became Empress consort upon her husband's accession as Tsar Nicholas I in December 1825 during a turbulent period marked by the bloody repression of the Decembrist revolt.
Split into simpler sentences: Alexandra Feodorovna became Empress consort upon her husband's accession as Tsar Nicholas I in December 1825. It was a turbulent period, marked by the bloody repression of the Decembrist revolt.