The aunt tried to run away and paid for it with her life, Hideko would rather commit suicide than be brought back into the basement - why?
The octopus is the answer. It tells us, what happened down there, without having to show it on screen.
From Philip Gowman: Park Chan-wook talks about Handmaiden, octopuses and more, London Korean Links, 2016.10.24:
Does Park have an obsession with the octopus?
No. The boneless sea creature that Choi Min-sik ate alive in Oldboy is 낙지. The writhing object in the basement in Handmaiden is 문어. Two different words in Korean. [...] In Handmaiden the Korean audience will associate the octopus in the basement with a famous Japanese erotic print. That means they know what happens in the basement without Park having to film what is depicted in the print.
The Oldboy octopus is the Octopus minor (nakji), a part of Korean cuisine. The Handmaiden octopus is the Giant Pacific octopus.
Miranda Chance wrote in Pleasure / pain and The Handmaiden -- does a film have to be difficult to be good? for thecontentjournal:
But not only is the octopus the director’s personal nod to Oldboy (2003), a cult classic, but it conveniently features heavily in Japanese pornography, the most famous of which, Hokusai’s ‘The Dream of the Fisherman’s Wife’, makes an appearance.
According to its wikipedia article,
it depicts a young ama diver entwined sexually with a pair of octopuses.
The picture itself is also featured in the movie, according to the Cannes 2016 Review.