Having Mars transiting your 8th or 12th natal house is not an easy period of time. the 8th and 12th are internal, intimate houses. Having Mars there is like having an elephant in a china store – mars shows its malefic side when entering any of these houses since naturally he is an energetic, external and aggressive planet, while these houses are delicate, receptive and passive.
Usually having Mars in either the 8th or 12th natal house indicates a sensitive time in which some deep fears, resentments, phobias, anxieties and intrigues arise.
The 8th house, being more partnership related, may arise fears and harsh feeling towards other significant people. Issues of trust are likely to harm the image of the other person and to interfere with otherwise smooth relationships. Self doubt is less likely here: the 8th house is about inner drive so with Mars here there might be a feeling of having no energy anymore. feeling drained or exhausted from life is likely with this transit.
The 12th house is more related to general phobias, depression and fears. A person with Mars transiting this house may feel overwhelmed by the world, may have problems communicating their feelings and sharing their fears therefore the result might be feeling isolated and alone. Sleeping problems and addictions may also be a cause for a more lasting depression when transiting Mars is in the natal 12th house.
These two houses are also connected to self strength so having Mars in any of them may result in feeling weak, helpless and hopeless. Mars in these houses tend to take energy instead of giving energy so the usual self strength, emotional ease, confidence in life that a person feels may turn into less favorable feelings.
Since mars transits a natal house for at least 2-3 month, the manifestation of its influence on the 8th or 12th house won’t be short and sharp. Instead, a sensation of isolation or mistrust will start build themselves from the moment Mars touches the cusp of the house. later on there will be a period of building up all the negative emotions, dealing with them and learning how to handle them, to a point that usually when Mars left the house, the person feels a huge relief, but not sudden and surprising one – rather a feeling that they themselves learned how to rule their fears.
Kabbalistic Feminist Astrology 
I had both of these this year. Mars is going to leave my 12th house tomorrow. The 8th house transit was more about problems with a couple of relationships that simply had to end and having to let go of my activist life completely — all of which turned out to be a GOOD thing. The whole Mars transit of my 12th house has been about learning and reacting to finding out that my small apt. livingroom shares a wall with 28 wireless smart meters. I went into weeks of compulsively researching how I could lower the other EMF radiation in my home, and have made some changes. But I still feel what you say: “overwhelmed by the world, may have problems communicating their feelings and sharing their fears therefore the result might be feeling isolated and alone.” I am looking forward to returning to a state where I feel more of my “usual self strength, emotional ease, confidence in life.” THANK YOU FOR THIS. I’LL SAVE IT, AND REMIND MYSELF IN THE FUTURE WHEN MARS IS TRANSITING THESE HOUSES. Sally
When my mars is on the 8th, i always have a lot of mental breakdowns, changes in my personality, fears, e become lunatic, breaking with reality . It’s hard to be strategic on this period. But every transit of mars in the 8th, i become someone new, with new ideas. Sorry the english, i’m south american.
I just read your interesting article about Mars transit 12th, which i currently have, and it covers 3 signs, yes a large house, it is in Pisces at the moment so am feeling the vagueness of it and definitely feeling a little under the weather today. and not doing much because i don t feel like charging around like i usually do. i have 2 holidays (thankfully ) during the 12th mars transit so will be able to chill out a bit and just go with the flow. My sleeping is a bit weird too, as i usually sleep like a top, and i am waking up so early but feeling tired later in the day. I will watch the mars transit as it goes on for until beginning of feb, cheers.
Its an introvert house, the 12th, while Mars is the most extrovert planet. Feb is right around the corner, hang on!