The Shiv Sena is a vigilante, Marathi chauvinist, Hindutva political organisation which has an electoral and ideological alliance with the BJP. Founded by Bal Thackeray in 1966, it has often been described as a fascist party. It has an extensive record of orchestrating communal pogroms primarily in Maharashtra: notably the 1970 Bhiwandi riots and the 1992-3 Bombay riots. Extensive participation of the Shiv Sena in these riots is documented in the Srikrishna Report.
An important part of their agenda involves moral policing/cultural terrorism. For example they have attempted to block Pakistani musicians and athletes from visiting India and in February 2014, they attempted to sabotage the formation of an Indo-Pakistani rock group. They have also imposed bans on films they consider "indecent", threatened film stars, and attacked journalists. They frequently attack couples on Valentine's day.
References:
• Articles on the Shiv Sena on South Asia Citizens Web, Communalism Watch and Narendra Modi Facts.