While the 7th House symbolizes the outside world manifested in marriage partners and enemies, the 8th House represents situations and circumstances that are borderline. Coincidentally, you may well feel like Madonna’s heroine from the song with the same title, namely:
Feels like I’m going to lose my mind…
However, human beings are remarkably keen to adapt, so our idea of what is borderline vs. familiar are very changeable. A notable exception is the direct threat of death, but even here there are complications when it comes to, say, soldiers in an active army or firefighters.
Situations of the Eighth House mean a change in the leading egregor or a position on the border of two egregors when a person must quickly reorient from one to the other or serve both at once, allowing them to communicate and exchange information through that person (which often poses serious challenges as egregors do not bother with human ethics).
In addition, the Eighth House governs group ethics, as opposed to the personal ethics of the Second House. This means that, while in the Second House you personally are the author of your rules of behavior in the world, the 8th House presents the problem of taking responsibility for the behavior of all the groups you are a part of. At a high level, the inclusion of the Eighth House means direct work in the egregor, which is associated with unforgettable mystical experiences and deep meditations.
In order to correctly interpret the synastric planets in the Diurnal Houses, one must bear in mind their sacrificial character. So, when your 8th House is active, you will always have (at least subconsciously) the feeling that you sacrifice yourself for the sake of the world, losing your energy and integrity, dying and disintegrating (symbolically or literally). At the same time, of all Diurnal Houses, the Eighth one gives, perhaps, the strongest and deepest experience of sacrifice, voluntary or compulsory. This feeling will definitely color your attitude towards your 8th House synastry partner, but it may well be that you will make sacrifices for him or her willingly and voluntarily – if the partner is able to adequately perceive what is happening.
The activation of your Eighth House by the planets of your synastry partner at first can lead you both into a strong delusion whose nature depends on the natal aspects of the 8th House. If the Eighth House is harmonious, the activation of its synastric planets can give you the impression that your partner is the person who’ll finally make you rich. In a more sophisticated scenario, you will gain an opportunity to directly contemplate God; at the intermediate level, you can fall in love with an Eighth House synastry partner: madly and briefly. If your 8th House is struck, misfortunes may follow and the tension of your life as a whole will sharply increase; at the same time, the option of madly falling in love with the partner remains open.
In this case, your probable delusions will mainly consist of you thinking the state of your partner’s mind to mirror your own, which, most likely, is not so. You may also hold this partner accountable for what is happening to you, which will also be completely unfair: he to she only activated your Eighth House, so some of your unsolved problems associated with egregors surfaced, and the defects of your group ethics emerged and showed its ugly face. The partner will be inclined to think that such extreme states of yours arise for his or her entertainment and that they have no direct relation to him, but this is not the case.
Your borderline states serve for your 8th House synastry partners as manifestations of their own unrealized problems. It is only a hint, true, but if the partner turns a blind eye to them these issues can hit the partner’s life as hard as they did yours. Working out the aspect and the relationship as a whole, however, will give you an intimate friend on whose support you can count even in most acute situations: he or she will do exactly what you need, regardless of whether you know what you need yourself.