Short answers, pulled from the story.
The Gesamtkunstwerk, or total work of art, was Wagner's vision of opera as a unified fusion of music, poetry, dance, and visual stagecraft rather than a sequence of alternating arias and recitatives. Wagner outlined this idea in essays published between 1849 and 1852, most fully realising it in Der Ring des Nibelungen.
Wagner played a minor supporting role in the May Uprising of 1849 in Dresden, and warrants were issued for the arrest of participants. He fled Germany and spent the next twelve years in exile, primarily in Zürich, before the political ban was fully lifted in 1862.
The Bayreuth Festival is an annual opera festival held at the Festspielhaus in Bayreuth, Bavaria, devoted entirely to staging Wagner's mature works. Wagner had the theatre built to his own specifications to realise his vision of total artistic immersion, including a sunken orchestra pit hidden from the audience and a darkened auditorium. The Festspielhaus opened on the 13th of August 1876 with the first complete performance of the Ring cycle.
The Tristan chord appears in the opening bars of Tristan und Isolde, composed between 1857 and 1859. Some music historians date the beginning of modern classical music to this chord, as the harmonic language of Tristan pushes tonality to its breaking point and is widely seen as a precursor to atonality in the 20th century.
Adolf Hitler was an admirer of Wagner's music and visited Bayreuth frequently from 1923 onwards. Wagner's music was used at Nazi events, and research suggests it may have been used at Dachau in 1933-1934. However, there is no credible evidence it was played at Nazi death camps, and one scholar has noted it was explicitly off-limits in the camps. Houston Stewart Chamberlain, who married Wagner's daughter Eva, wrote a text approved by the Nazi movement, though he never met Wagner himself.
Der Ring des Nibelungen consists of four works: Das Rheingold, Die Walkure, Siegfried, and Gotterdammerung. Wagner worked on the cycle for 26 years, from the first libretto draft in 1848 to the completion of Gotterdammerung in 1874. The complete cycle runs approximately 15 hours in performance.