The story of Pandavas and Rakshasa Kirmira happens during the exile period of Pandavas in the Mahabharata. The story is narrated by Rishi Maitreya to Kauravas. The reason for narrating the story was to make the Kauravas aware of the power and strength of Pandavas. The story is found in the Vana Parva of Mahabharata.
When the Pandavas entered the Kamyaka forest, they were attacked by Rakshasa Kirmira.
This Rakshasa was troubling all the living beings in the Kamyaka forests. Saints and human beings were unable to conduct their daily routines.
Bhima took on the Rakshasa Kirmira. They both fought with hands and by uprooting trees. The Rakshasa then took huge boulders to attack Bhima. But Bhima powdered the huge boulders using his mace.
Might Bhima then stuck the Rakshasa down with his mace and pinned him down to the ground.
He then snapped the neck of the Rakshasa killing him instantly.
All the residents of the Kamyaka forest came out of hiding and prayed for the welfare of the Pandavas.
Sub Story - Rakshasa Kirmira had a sister named Simhika. She had attacked Draupadi but got mutilated by Sahadeva. So it is believed that Rakshasa Kirmira attacked Pandavas to take revenge.