The Knights' Room was a large hall at Hogwarts Castle, with many lined-up knights. There was an archway that divides the space into two areas.  It is located on the Seventh Floor Corridor.
In 1991 and 1992 the knights were bewitched by Professor Quirrell (acting for Lord Voldemort) to be evil and attack Harry Potter..
In 1992 and 1993 the knights were bewitched by Dobby to attack Harry Potter so that he would return to Privet Drive (trying to prevent him from being killed by the Chamber of Secrets Basilisk).
The knights in this room, along with the others in Hogwarts Castle, participated in the Battle of Hogwarts after being animated via Piertotum Locomotor by Professor Minerva McGonagall.
Notes and references