Milady is now the prisoner of the brother-in-law she would like assassinated, so her son would inherit his wealth.
But D'Artagnan and the three Musketeers have gotten message sent to him telling him of her villainy, including plot to assassinate Duke of Buckingham. The word "fauteuil" is mentioned.It means armchair, but I looked it up at:
I found a few interesting words. For instance, when is something to eat also something to sit on?
When it's a canapé. The glossary above says it's a sofa,
my dictionary says it's an appetizer to be served with drinks.