Places
Forbidden Forest
The Forbidden Forest stands on the edge of the Hogwarts grounds. Students are not allowed in it. It holds unicorns, centaurs, Aragog’s descendants, and the thestral herd. Harry serves a detention there in his first year, meets Aragog in his second, and walks into the forest in the last book to meet Voldemort.

First mention
Harry Potter and the Philosopher's StoneChapter: The Forbidden Forest
Harry, Hermione, Neville, and Draco serve detention with Hagrid and see a hooded figure drinking unicorn blood.