Don't expect a smooth, calm, uncomplicated relationship, because with this aspect between you, it's anything but! Your lover challenges you on the deepest of levels and they draw out all your darkest emotions. This person challenges you to face your past, your roots, the forces that you came from that were beyond your control but which shaped you and made you the person that you are today.
Needless to say, this can be an uncomfortable, difficult, even painful process for you! You might discover things about your past that you just don't want to think about, much less deal with. This aspect can have a really cathartic effect, because it forces transformation. The person you once were, operating in the world based on a series of influences, experiences and encoded lessons, doesn't necessarily have a whole lot to do with the person you become once you work through all those old, dark feelings. Your lover helps you to understand that truth, and to go through the transformation that needs to happen. You become a brand-new person as the result of this relationship, and that is a good thing -- even if it doesn't feel like it in the moment. You should take this as an opportunity to peel away at the layers of your subconscious mind like an onion, and you come through it stronger, more self-aware and emotionally empowered.