Which composer wrote the Symphonie fantastique about an artist's opium dream?

Berlioz, in 1830, at 26 - and the programme he printed with it is autobiographical to an embarrassing degree. A young artist, in love with a woman who does not know he exists, takes opium and dreams that he murders her, is executed, and attends his own witches' sabbath at which she appears as a grotesque.

The woman was real. He had seen the Irish actress Harriet Smithson play Ophelia and Juliet in Paris, conceived an obsession, written her letters that alarmed her, and been refused a meeting. He then wrote a symphony about killing her. She attended a performance two years later, worked out that she was the subject, and married him - and the marriage was a disaster.

The music was decades ahead of anything around it. It gives the beloved a recurring theme, the idée fixe, that transforms across all five movements according to the narrative - which is the technique Wagner later built into the leitmotif and every film score since. Berlioz also wrote for an enormous orchestra, invented instrumental effects nobody had used, and wrote the orchestration treatise that Mahler and Strauss learned from.