Chapter 9 - "Grim Defeat" - The students are gathered in the Great Hall for the night, Quidditch practice continues, DADA is taken over by Snape when Lupin falls ill, Peeves wakes Harry, Quidditch match between Gryffindor and Hufflepuff in a thunderstorm, Harry sees the Grim and Dementors on the pitch and passes out to awaken in hospital having lost the game.
Hogsmeade, Halloween, and a Shredded Portrait
The students all enjoy a lovely Halloween feast, complete with gliding formations by the [Hogwarts ghosts], and then head back to their dormitories.
The Gryffindors all stop outside Gryffindor Tower, discovering that the Fat Lady is gone and her portrait shredded. [Professor Dumbledore] arrives to sort things out, and quickly learns from Peeves who the culprit was - Sirius Black.
Sleepover in the Great Hall
[Professor Dumbledore] gathers all of the students into the Great Hall, leaving them sleeping bags and orders to alert him with any news, as he heads off to search the castle with the other teachers.
Harry, Ron, and Hermione lie discussing Sirius Black's sneak attack on the Fat Lady. They then hear [Dumbledore] arrive and tell Percy that Black is gone and the Fat Lady has been found - and overhear Snape expressing concern about some Hogwarts teacher who might be helping Black in. We also learn of Dumbledore's immense dislike of [dementors].
As time passes, students gossip about how Sirius Black could have entered Hogwarts, Sir Cadogan replaces the Fat Lady outside Gryffindor Tower, and teachers (along with Percy Weasley) begin staying annoyingly close to Harry at all times.
Preparing for Quidditch
Out of her concern for Harry's safety, Professor McGonagall tries to tell Harry to stop attending [Quidditch] practices - but at his protests, she relents and instead assigns Madam Hooch to be present at each practice.
As the first Quidditch match nears, the [Gryffindor team] learns that they will be playing Hufflepuff instead of Slytherin, owing to Malfoy's fake injury. [Oliver Wood], fearful of what this could mean for their chances, struggles to keep the team serious about the match.
Harry struggles to focus on his classes as the [Quidditch] match nears and storms howl outside - especially as [Oliver Wood] makes him ten minutes late to Defence Against the Dark Arts discussing Cedric Diggory's swerve.