An oratorio is a large-scale piece of music for choir, orchestra and vocal soloists, which tells a story without staging or costumes. Most oratorios are based on stories from the Bible, a source that Handel's English audiences knew well.
Enthir is a Bosmer sorcerer and merchant found at the College of Winterhold, and has connections to the Thieves Guild in Skyrim.
Activating the quest. As you travel through Skyrim, you might want to visit the south-west region of Riften. There, you should look for Froki's Shack. Finding it isn't easy, so check the map below. Enter the dwelling and you should see lots of trophies, an old man and a child.