This is Trishul (Trident), Siva’s primary attribute. The Trishul serves Siva as a weapon for cosmic destruction. Some consider this a weapon grace, more like a weapon of deterrence. The Trishul is worshipped on its own and used as a substitute for Siva in some of the processions. 

This Trishul is about 14 cm in height, not including the base. At this size, it is likely to have been mounted on a pole. The center is hollow. This Trishul is devoid of any decoration, but for the curved outer prongs and the bands on the stem. 

Some of the Trishuls, especially dating to the 10th-12th century (Chola period) have figures attached to the central prong. 

Strangely enough only very few of the processional Siva images carry Trishul as one his attribute. The three prongs are meant to signify three qualities purity, activity and lethargy.

Trishul is also carried by Mahishasuramardhini, Bhairava and Bhairavi.