If Mars is in 12th House, then The native will suffer from want of finance. He will suffer from rheumatic and bilious diseases. If Mars is associated with a malefic the native will have false vanity.
The Twelfth House has a terrible reputation or losses . It is here that astrologers look to find restrictions, hidden enemies (as opposed to open enemies, shown by the seventh house), psychological problems, and of course the killer --- self undoing, the tendency to do ourselves in. Any planet in this house represents something we are hiding from the world, and, frequently, from ourselves as well. Of course knowing about a problem is the first step to deal with it. The rest are horoscopes of serial killers and mass murderers.
Mars here in bad position makes native lose lands & Conveyances and also loss of Brothers.
Mars in 12thH people hides their anger and aggression. Watch out for this person when their temper finally explodes.
Other Views
Brihat Jatak- The person will suffer degradation.
Phaldeepika- The native will be a backbiter, cruel, without wife and mean. He will also suffer from eye troubles.
Saravali- The native will suffer from eye troubles, he will be sinful, mean, killer of his wife, backbiter and cruel, and will be liable to get imprisonment.
Chamatkar Chintamani- The native will be irreligious and a liar. He will have fear from thieves.