Pandavs had lost the gamble which they played with Kauravs. According to the conditions of the gamble, they had to stay in forest for 12 years.
During their stay in the forest Bheem, one of the the Pandav brothers, fell in love with a demon named Hidimba. He married her with permission of his mother Kunti. Ghatotkach was born out of their marriage. As Ghatotkach was born half demon he had extraordinary powers.
There is also a story of Karn connected with Ghatotkach. Indra had cheated Karn and had taken his heavenly armor and earrings as donation. In return he had given an arrow called Shakti to Karn. Karn could kill anybody with that Shakti.
Lord Krishna invited Ghatotkach to fight in the Mahabharat war. Ghatotkach kept killing thousands of Kaurav soldiers everyday. Ultimately he started killing so many soldiers that Duryodhan requested Karn to use the Shakti on Ghatotkach. Though Karn had saved the Shakti to kill Arjun, he had to use the Shakti to kill Ghatotkach on the insistence of Duryodhan. When Ghatotkach died he increased his body with his demonic powers and killed thousands of Kaurav soldiers when he fell down.
Thus Ghatotkach saved the life of Arjun and brought a lot of destruction on the Kauravs army.
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