Sun in 8th House synastry relationships are largely of business and corporate nature. Such partners don’t necessarily communicate with each other directly: they can be in the middle of a complex cycle within a common chain of a business process. If a direct relationship does arise, common hobbies include occultism and psychology.
Under certain favorable comparative aspects and a number of other circumstances related to the area of hidden human feelings, the emergence of intimate relationships is also possible. A person with a higher Solar energy potential can very successfully stimulate another person in many aspects of life, taking into account the peculiarities of the Eighth House, ultimately leading to their joint activity and coexistence.
Negative aspects of Sun in the Eight House synastry not only can distort and complicate this type of relationship but also end in a noisy conflict with subsequent separation.
from F. Sakoyan, L. Ecker
Sun in 8th House Synastry Explained
If the Eighth House is stricken, you may perceive your partner as a sworn enemy; at least many of his initiatives will make you shudder. A closer look will show you, however, that the partner is rather a catalyst than a source of your stress and crises. Moreover, the partner may not have such effects in mind at all, especially if his Sun is harmonious.
It would be a big mistake on your part to start an open (and even more so covert) war with a Sun in 8th House synastry partner. Here, you are quite vulnerable, but this is not even the point: the main lesson that karma teaches in this case is where the flaws in your group ethics are. For example, you must understand the true extent of responsibility for your partner and both of you.
The Sun in Eight House synastry partners, on the other hand, should learn to perceive the states of your mind that arise as a result of their influence on you. If the partner’s Sun is in the Second House of their natal chart, at first it can be very difficult for you to understand each other: you will perceive the partner’s peaceful initiatives as the sound of an impending battle; but if you understand that their personal ethics, intertwining with yours, help you develop your own group ethics, then your relationship will become easier.
Nevertheless, an unexpected appearance of a Sun in 8th House synastry partner will always cause you a certain inner tension and a feeling of anxiety. If your Eighth House is developed, many of its situations will be extraordinary in the sense of taking responsibility for someone else’s fate. Then the partner him- or herself can act in the role of someone in dire need of help, or guide you into drama; but in any case, a high level of internal tension will be reached.
from A. Podvodny