Suddenly, light started shining through the window!
The window, which hanged on the wall, softly letting its curtains dance around the room, suddenly brought a bright light into the house.
Quite unexpectedly, light shined through the window in the room, which was less surprising when you think about the fact that's what windows are for.
Light shined through the window abruptly.
abruptly, usually means unexpected, or sudden. For instance, if your mother picked you up from school after telling you twice about doing that, it would not be abruptly. However, if someone were to tell you your house burned down and your parents were dead without telling you to sit down first, it would very much be called, abruptly.
A list of supernatural beings in the British Isles, from the Denham Tracts, 1892-5 (pictured).
~from The Penguin Book of English Folktales, Neil Philip, 1992
The author notes that this is where Tolkien found the creature name: Hobbits. I also see Fire Drakes. And I’d add that since this was published, there are at least two recognizable creature/character names J.K. Rowling may have gotten from it.