Towards the end of the Mahabharata, the war between good and evil, Dhuryodhana, who had opposed the courageous, kind-hearted Pandavas (the five sons of the king, Pandu) in the war, decided that he wanted to avenge his brothers' deaths by murdering the Pandavas.
How dare you murder my sons for no reason?!
To do this, he called his friend Ashwatthama, who had fought on his side during the war, and told him to kill the Pandavas in their cottage while they are asleep.
Krishna! My sons have been killed by Ashwatthama!
What?! Where is he?
That night, when it was pitch dark, Ashwatthama killed five men in the Pandavas' cottage who he thought were the Pandavas.
I hope you have learned your lesson!
However, to his surprise, Draupadi, the wife of the five Pandavas, caught Ashwatthama red-handed... and also told him in tears that it was not her husbands whom he murdered, but her sons: the Upapandavas!
Ashwatthama now felt ashamed of himself for murdering men whom he did not hold anything against, but was also afraid of the repercussions of his sin that were soon to come. Meanwhile, Draupadi woke up her husbands and rushed to seek the help of Krishna, who was actually a form of the god, Vishnu.
Krishna was enraged after hearing what Ashwatthama had done. To punish him, he forcefully pulled Ashwatthama's third eye that Ashwatthama had been born with, and inflicted a curse upon him that he should never be able to die. Hence, it is said that till today, Ashwatthama can be seen lurking the forests of India, with the scar on his head from his third eye being removed still paining him.