This is the only form of Lord Shiva in which He appeared black in color. arjuna wanted to get the peerless weapon pAsupatAstram from Lord Shiva for the battle of mahAbharata.

  This is the only form of Lord Shiva in which He appeared black in color. arjuna wanted to get the peerless weapon pAsupatAstram from Lord Shiva for the battle of mahAbharata. He left the other four pANdavAs, went to the forest and did austere tapas for getting the boon of Lord Shiva. Lord Shiva was satisfied with his prayer, wanted to give the pAsupatAstram to him, but wanted to play a bit too. He took the form of a Hunter with Shakthi as Huntress. That time a demon by name mUkAsura, who was in the form of wild pig came to kill arjuna. To kill the wild pig arjuna fired an arrow from the front, at the same time the Hunter, Who is none other than God Himself, attacked it from the back and killed the pig.

arjuna mocked at the Hunter for firing the arrow from the back. Having great pride of his valor, he was als angry at the Hunter because He aimed at his prey. The Hunter responded that attacking an animal from the back is not against rules of hunting. An argument broke out. They decided to fight deciding who was more valorous between them. The Hunter cut the string in arjuna's bow with His arrow in the fight! Angered and excited, arjuna started wrestling. He couldn't match the Lord, and the Lord enjoyed his fighting. At one point of time in wrestling arjuna held the foot of the Hunter. As the Lord is pleased when somebody catches His holy feet, He stopped wrestling appeared with pArvati revealing Who He is. Shocked arjuna pleaded for forgiveness, as he was trying to fight with the Supreme out of his ignorance. However the God, Who is pleased by devotion, blessed him and gave him the invincible pAsupatAstra. (In some books it is told that arjuna couldn't fight with the Hunter, he started worshipping the Shiva Lingam. To his astonishment he found the flowers he offered to the Lingam on the head of the Hunter. Then he prostrated before the Hunter and the Lord revealed Himself). This can be found in detail in kirAtArjuniyam of bhAravi and mahA bhArata.