Mars in 12th House synastry relationships are based on activity at the subconscious level. The “Martian” personality excites the subconsciousness of the Twelfth House person. If Mars is negative, this causes activation of unpleasant and even painful memories causing unwanted emotional-sensory reactions. In this regard, the person of the Twelfth House considers the “Martian” to be rude and intrusive while she, in turn, considers the personality of the Twelfth House impractical, hovering in the clouds, indecisive, weak, and morally unstable.
With very high negativity of the aspect, real psychological cruelty can be expected from one or both sides. Latent outright hostility is possible. With a positive aspect, the partners are able to calmly resolve any conflicts without prejudice and continue a positive life associated with charity and religious and spiritual activity, as well as with occultism and psychology.
from F. Sakoyan, L. Ecker
Mars in 12th House Synastry Explained
In a positive version of Mars in 12th House synastry, the activity of the partner will direct you to the path of service – which one, the position of the 12th House in your natal chart will show. At some point, the partner will become the object of this service and may require sacrifices from you, promising something intangible but perhaps very attractive to you in return.
At a high level of spiritual development, a Mars in Twelfth House synastry partner will be able to give you a lot, but only after your preliminary purification – by sacrifices and repentance in the first place. At a low level, his ruthless exploitation of your strengths and hopes is quite likely. In the 12th House, karma is unleashed, often going back to past incarnations, so your sufferings brought on by this partner may seem to fall out of your general life story.
Mars in Twelfth House synastry partners will help you develop genuine humility and the ability to see any experience as a valuable and unique opportunity to participate in spiritual evolution by personal self-denial and to directly feel the response of the Universe.
from A. Podvodny