The dense fog filled the room so that Harry could not see Slughorn or Voldemort at all; only Dumbledore, smiling serenely beside him.
"Bill told me W Fred and George are very amusing!" said Fleur, smiling serenely.
"I enjoyed the meetings too," said Luna serenely.
The ceiling of the Great Hall was serenely blue and streaked with frail, wispy clouds, just like the squares of sky visible through the high mullioned windows.
"You’re quite right," said Dumbledore serenely, shaking back his sleeve to reveal the tips of those burned and blackened ringers; the sight of them made the back of Harry’s neck prickle unpleasantly.
And Harry Potter’s now having an argument with his Keeper," said Luna serenely, while both Hufflepuffs and Slytherins below in the crowd cheered and jeered.
There’ll be panic, more parents takin their kids outta school, an nex’ thing yeh know the board o’ governors …" Hagrid stopped talking as the ghost of a long-haired woman drifted serenely past, then resumed in a hoarse whisper, "…. the board o’ governors’ll be talkin about shuttin’ us up fer good."
There are no more uses of "serene" in the book.
Show samples from other sources
While I lie awake worrying, she sleeps serenely.
She remained serene even when he was yelling at her.