Karna, refused by Drona, goes to Parashurama pretending to be a brahmin and learns the Brahma weapon. When a worm bores his thigh while his teacher sleeps on his lap and he does not flinch, Parashurama knows he is no brahmin and curses him to forget the weapon when he needs it most.
Drona will not teach Karna the Brahma weapon, so Karna goes to Rama on Mahendra and says, I am a brahmin of Bhrigu's race, and is welcomed. Wandering the shore, he kills a brahmin's homa cow without meaning to; the owner will not forgive him and says the earth will swallow his chariot wheel when he fights his enemy. Later, Rama, weak from fasting, lays his head in Karna's lap and sleeps. A worm with eight feet and needle bristles, Alarka, bores into Karna's thigh. He bears it rather than wake his teacher. The blood reaches Rama, who wakes: I have been made impure. The worm dies under his eye and an asura rises, freed from Bhrigu's ancient curse. Then Rama turns on Karna: no brahmin could have borne that pain. Karna confesses he is a suta's son. Since you lied for a weapon, says Rama, it will leave your memory when you face your equal. Go.