The relative pronoun which is understood as the object of wrought. It refers to "the corporeal".
We can understand your sentence as a combination of two smaller sentences:
She is held fast by the corporeal. The continual association and constant care of the body have wrought the corporeal into her nature.
To put them together, we replace the second instance of "the corporeal" with the relative pronoun which. Because we have to move which to the front of the relative clause, it leaves behind a gap:
[ which [ association and care ] have wrought _____gap into her nature ]
When we read this sentence, we understand which as the object of wrought, even though it doesn't appear in object position. In syntactic terms, we can say that the gap is the object, and which and the gap refer to the same thing; they both refer to "the corporeal".