The Eighth House, ruled by Scorpio, represents birth, death, deep emotional connection, fears, and inheritances. The astrology joke is that the eighth house rules sex, taxes, and death - a fertile ground for fears and obsessions!
The eighth house is also where we merge with something greater than ourselves. In sex, you merge with another such that two becomes one. With taxes, you merge with the collective unconscious of what USA readers might call the great Uncle Sam tax god. In death, you go beyond all boundaries and merge with Oneness. In a relationship, the eighth house talks about how you share your body, your time, your possessions, and so forth with your partner.
It represents your partner's income, accounting, insurance, and financial planning. Depending on the planets you have in this house, it may indicate steady income, or constant windfalls and debts.
Emotionally, this area is ruled by the planet Pluto (death, obsessive-compulsiveness, power, regeneration, elimination). The eighth house is where you go into the swamps of those intangible fears. The eighth house helps you to get in touch with the deepest feelings that you carry below the surface.