This is the classic Christmas piece "Carol of the Bells," composed by the Ukrainian composer Mykola Leontovych (based on the Ukrainian folk song "Shchedryk").
The version in the video you're watching is a piano arrangement by William J. Ross.
This is the best I've heard any person articulate the "why" of The Pendragon Cycle.
Thank you, Glenn. Truly.
And I accept your criticism of the realism in that particular scene. It's my failure, and I own it.