Schoenberg started writing
Moses und Aron (note he did not use Aaron because he was superstitious about a 13 letter title) shortly after WWI in response to the rising tide of anti-Semitism in the German speaking lands. And while it was still unfinished at the time of his death in 1951 it is considered one of, if not the great masterpiece of his career. I have enjoyed his earlier, shorter, smaller, expressionistic "opera" (technically a monodrama because there is only one character)
Erwartung but Moses is a full fledged unapologetically 12 tone opera so I have avoided it until now.
That was a mistake. This is fascinating, impressive and deeply affecting music. It is the story of Exodus from the Old Testament, and focuses on the relationship between Moses and his brother Aaron. And while it is, I guess, academically 12 tone, and the "melody" and harmony are sometimes aggressively modern, listening to it was not unpleasant at all. It just takes more concentration than conventional tonal music because there is no recognizable melody or chord progressions.

Herbert Kegel was a modern music specialist based in the old East Germany who made a bunch of really good, but largely neglected recordings. There is the issue of the final act, which was not set to music and is not in this recording. It is often presented as spoken dialog, but while it is public domain in most of the world it is under copyright in the US and I can't find it online. I'll probably buy the score.