After killing the victim, the family had buried his body in the septic tank in their backyard.
The incident came to light on Friday after the neighbours got suspicious and informed the police.
The police has recovered the body and arrested four of the accused, while another is still absconding.
According to the arrested family members, the deceased, Uttam Sutradhar, was mentally unsound, did not work but always quarrelled with others at home.
An investigation is underway. (ANI)