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.
Installation: Place the contents of the archive in your Skyrim/Data directory. You can install this mod at any time: before you start the Karstaag quest, during the quest, after the quest, even after you have already summoned him three times and lost the power.