Aragog is a fictional character from the Harry Potter books and movies. He is an acromantula or giant spider. He makes his first appearance in Harry Potter and the Chamber of Secrets and later comes back in Harry Potter and the Half-Blood Prince. In the Harry Potter films he is voiced by Julian Glover.
In the book, Harry Potter and the Chamber of Secrets it is revealed that he was brought to the UK as an egg by a traveller and raised by Hagrid while Hagrid was in school, but Tom Riddle framed Hagrid for the death of the girl (Myrtle) the first time the chamber was opened. Aragog is blamed by Riddle as being the monster from the Chamber of Secrets and as a result Hagrid is expelled and Aragog is forced to flee into the forest where he lived for many years until he died Harry Potter and the Half-Blood Prince.
Aragog makes is first appearance in Harry Potter and the Chamber of Secrets when Harry Potter and Ron Weasley are told by Hagrid to "follow the spiders". They do so and the spiders lead them into Aragog's lair. Aragog gives Harry and Ron clues as to who opened the Chamber of Secrets the first time, leading them to realise that Moaning Myrtle was the original victim of the Chamber. He also tells them that Hagrid didn't open the Chamber.
Aragog makes another appearance in Harry Potter and the Half-Blood Price. Harry is told by Dumbledore to procure a memory from Horace Slughorn. To assist him in procuring the memory Harry drinks the Felix Felicis potion which makes him extreamly lucky. The Felix Felicis guides him down to Hagrid's Hut but on the way he runs into Slughorn. Harry then suggests to Slugorn that he should come down to Hagrid's Hut with him to attend Aragog's funeral. Slughorn agrees because he wants to see if he can get some of Aragog's venom as it is extremely rare and valuable.
At Hagrid's Hut they find Hagrid mourning over the death of Aragog. After a small ceremony, Hagrid, Harry and Slughorn go inside. Hagrid and Slughorn get quite drunk resulting in Harry procuring the memory from him.