This form of Mahakala is known as the Nagpo Chenpo meaning the great black one. Mahakala is one of the protector of Buddha. In his right hand he holds a sword or a khatrika. In his left hand he holds a kapala, a skull full of blood which is used for some prayers in Tantaric Buddhism.
This yak bone carved is about 6 inches long and 3 inches wide. The weight is around 300 gms. The age is about 30-50 years old.