Rahu in 12th House. 

So, lets see what Rahu can do in 12th house without any conjunction or aspect from any other planet. 1st lets see what these 2 things represent - 

Rahu represents illusion, foreign lands, foreign things (foreign as in unknown things. So, for someone living in Punjab, culture and rituals of Kerala is also unknown and hence Foreign), Drugs, Medicines, any type of intoxication, fame, wealth, success, obsession, past-life left-over karma, movies, television, online world, cheating, imagined fears, unusual things, unique things, creativity, rule breaker. Rahu also blows things out of proportions. 

12th house is house of Losses, Expenses, Isolated Places, Spirituality, Foreign Lands, Foreign Companies, Imagination, Sub-Concious Mind, Charity, Donation, Asylums, Jails, Hospitals, Hidden Talent and Secrets of Other World etc.

One thing to always keep in mind with 12th house is that it basically remains the house of losses. So, whichever planet goes in 12th house, even exalted, is in the bucket of losses. Now, to gain out of any such planet, you need to put double effort in things related with that planet.

Aspect-wise, in Vedic Astrology, Rahu has 5th and 9th house aspect. He is unable to impact 7th house from its place as Ketu is sitting there, which is the other axis of Rahu and represents all those things which are non-Rahu. 

So Rahu in 12th house, 1st of all gives obsession towards reaching foreign lands as Rahu represents Foreign Things and 12th house itself is foreign lands. Due to its malefic nature, Rahu takes the person away from his home into foreign lands. This position can also show someone who is spendthrift. This position can show different type of careers. But best indication is - As 12th house is also a house of Spirituality, it also shows someone who is immensely obsessed about spiritual pursuits.  The end result depends on the sign Rahu is sitting in and also conjunction of Rahu with other planets. 

From 12th house, Rahu aspects the 4th house of home, home environment and mother etc and 8th house of occult, mysticism etc. As all these houses are related with Moksha or spirituality, it becomes a great position of spiritual nature. That's why, spiritual obsession is mainly seen here. 

Hope this helps. 

Thanks,

Swami Premanand Bharti