When Mercury is placed in the 8th house in the Libra sign in a person's birth chart, it indicates a person who is inclined towards deep psychological and metaphysical subjects. This placement tends to give individuals an interest in occult sciences, astrology, and spiritualism. The native possesses excellent analytical and research skills, which assist in understanding deep and hidden secrets of life. 

However, the placement of Mercury in the 8th house also indicates that the person may face some stressful situations in life related to debts, taxes, inheritances, and joint finances. These issues may cause mental agony and anxiety for the person.

This placement also tends to give the native a sharp and clever mind, which can help in business and financial dealings. The person may possess excellent persuasive skills, which can help them in their profession.

Overall, Mercury in the 8th house in Libra sign is a significant placement that can help one attain success in their profession and a keen interest in spiritual matters. However, the person must take care of the issues related to debts and financial problems.

Mercury rules 4th and 7th house is in 8th house in Libra