Mars in 12th house is both good and bad. Depending on the
position of other planets it might burn malefic. Here are the results or
effects of Mars in 12th house.
Will have good spouse.
Will have leadership qualities.
Will have good children.
Siblings will be helpful.
Mars in 12 and Ketu in 3 is considered highly meritorious.
The person will be rich and powerful.
Mars in 12 and Sun in 3 or 11 is highly beneficial.
Wearing red ruby will be helpful. Donating red color things
are also useful.
Note - Depending on the placement of other navgrahas, a
person might not suffer all problems. Sometimes all the good might turn bad due
to bad positioning of other planets.