This (mostly) serene yet powerful and profound piece of music was composed by Richard Wagner sometime between 1845 and 1848.

Here it is, with an orchestra conducted by Simon Rattle (9 minutes).

Here‘s the Wikipedia entry on the whole opera.

Here‘s the synopsis on the same page.

Summary of the synopsis:

The people of the Brabant are divided by quarrels and political infighting; also, a devious hostile power left over from the region’s pagan past is seeking to subvert the prevailing monotheistic government and to return Brabant to pagan rule. A mysterious knight, sent by God and possessing superhuman charisma and fighting ability, arrives to unite and strengthen the people, and to defend the innocent noblewoman Elsa from a false accusation of murder, but he imposes a condition: the people must follow him without knowing his identity. Elsa in particular must never ask his name, or his heritage, or his origin. The conspirators attempt to undermine her faith in her rescuer, to create doubt among the people, and to force him to leave.