Mars represents our life force, the fire within that pushes us to get up in the morning, pursue a mate, a job, a career, work on our goals. Without Mars we don’t exist in the physical world, without Mars we’re only thoughts and emotions. Mars is ruled by Aries and anciently, by Scorpio. The 8th House being the House of Scorpio, this is a powerful placement to have, but not necessarily easy.
When Mars is located in the 8th House, there is a very deep and powerful inner power. The person will keep on going and trying when everyone else dropped the ball a long time ago. They have a power of endurance, a deep-rooted energy that can carry them wherever they want to go. They can have a deep anger, a burning rage and anxiety that can lead them towards obsessive and controlling behaviours.
Like with any other 8th House placement (see Moon in Scorpio or Venus in 8th) there is a deep-rooted anxiety, a fear, and the origin will be depending on the sign and aspects of Mars. This anxiety would come from a subconscious place, and the person could need some time and maturity to come to terms with it.
Mars located in the 8th house is deeply interested with knowing thyself, the energy is dedicated to transforming oneself, over and over, until layers of pain, trauma and suffering are gone. By essence, someone with this placement is drawn to psychoanalysis and drawn to facing their own shadow. We might also find people deeply drawn to esoteric and Astrological studies. With Mars in this house, you're either facing your shadow willingly, or your own shadow catches you.
With this position of Mars, one can experience powerful forces - we could see people feeling like they have demons chasing them, or that there is a black cloud following them. I don't subscribe to these beliefs, however with Mars in our house of psychoanalysis, transformation, and our own personal shadow, if one doesn't actively seek self-knowledge and self-actualization, one own anger and rage will attack them from inside, like an auto-immune disease, hence the usually natural desire to seek enlightenment about oneself.
The 8th house is associated with sexuality, but it's a bit of a shortcut we make. 8th house is about intimacy, sexuality being - sometimes- one of the ways people build intimacy. I say sometimes because the culture we live in involves plenty of sexual interactions that have absolutely no emotional intimacy. Someone with Mars in the 8th house might seek intimacy in the wrong places (sexual encounters possibly) while they might shy away from actual intimacy - especially if Mars is challenged.
The purpose of this position of Mars though is emotional bonding and intimacy, and as you might have heard before - you can only know others as much as you know yourself - so if you have this placement and are in tune with yourself, you might have naturally been on a journey to know yourself. The more you indulge in such a journey and the more satisfying your personal relationships will be.
I have noticed some people experiencing deep-rooted and unconscious rage with this placement; it's in no way a curse but it's a potentiality that comes with it. The more aware you can become of your own emotions, the more likely you are to notice if you have anger and rage eating you from inside and driving you to act in ways that aren't beneficial for you.
Recommended books for Mars in the 8th house:
7 layers of intimacy: Very comprehensive book about what intimacy is and isn't and how to assess your relationships' level of intimacy
Daring greatly: How to use vulnerability as a strength
Betrayal bonds: Recognizing and breaking free of toxic and exploitative relationships
The subtle art of not giving a f*ck: A best seller book about how to be happy and how facing death helps you live