The 8th house covers everything that isn't purely yours anymore: shared finances, inheritance, deep intimacy, and the kind of transformation that only happens by trusting or merging with something outside your own control.
It's a succedent house, opposite the 2nd house of personal resources and self-worth, and describes the point where holding on stops working and the real lesson becomes learning what to let go of.
The 8th house traditionally governs shared and inherited money, a partner's resources, debts, taxes, and legacies, along with death, and by extension whatever gets transformed through loss, surrender, or merging with something beyond a person's own control. It's also long associated with sex, the most literal kind of merging with another person, and with the occult, whatever stays hidden until it's deliberately uncovered.
Opposite the 2nd house of personal, self-managed resources, the 8th house removes the assumption that value can be defined alone: resources here are entangled with someone else's history and decisions, which means trust, not effort, becomes the operative currency. That's why transformation is the right word for what this house does, less self-improvement than what happens once something a person was relying on stops being available in the form they expected, and they're left to find out who they are without it.