Arjun finds out

                                    

                                          
On the eve of the war, Karna had not even entered the Pandava camp when his enemy appeared.
"I fail to see, King of Anga," said Arjun in exasperation, "what exactly you hope to achieve by lurking around our camp every night. We do not discuss war strategies out in the open where any weasel can listen in..."
Arjun raised his divine arrow, the Gandiva. 
"...we are forbidden from killing after sundown, or before the war starts..." he continued.
He nocked an arrow.
"...and you have already issued me an impressive threat yesterday, so surely you would not feel the necessity to do that again today..."
Karna, who had come without his bow and arrow, reached for his knife. No sooner had he made the minutest movement than Arjun thrust the tip of his arrow an inch from his eye.
"Have you come for pre-battle practice?"
"No," said Karna.
"Very well," said Arjun. "You came into our camp. We will not allow you to leave before you state your intentions."
"I-" Karna glanced down at the ground and up again. "-did not enter your camp. I am on neutral territory and you cannot hold me captive, Arjun."
Arjun glanced down, too, and appeared most disappointed that Karna was, in fact, standing just outside the boundary of the camp.
"Fair enough," he said. "Get lost now. See you on the battlefield tomorrow. I will look out for you," he added with an arrogant and crooked smile.
Shaking with fury, shaking with hatred, Karna turned away to leave. If he had been a little calmer, he would have heard the footstep behind him.
*****************
Kunti met him near the boundaries of the Kaurava camp.
"Where had you gone, son?"
"I am not answerable to you," said Karna brusquely, brushing past her, but remorse overtook him instantly. In spite of the way his biological mother had treated him once long back, he could not help respecting and loving her for who she was, who she had always been, the kind-hearted and all-giving Queen of Hastinapur.
He should not take out his anger on Arjun on her. So he spoke without facing her.
"Now I know why you chose to tell me and not them, Queen Mother," said Karna bitterly. "You knew I would not be able to tell them, and you knew I would be a weakened opposition against your real sons."
"You are my real son as well, I am not taking sides, Karna-"
"Yes, that is visible."
Kunti reached out to lay her hand on her firstborn's shoulder.
"Believe me when I say I did not tell you because I wanted to weaken you, son," she whispered. "I wanted to tell them, too. You were the one who forbade me."
"Well, I take that back! Go and tell Arjun!" shouted Karna. "Go and tell Arjun, just to make it a fair competition!"
"Tell me what?" asked a dreadfully familiar voice behind them.
All the fight went out of Karna as he saw his mother's face going stark white. He turned, resisting the urge to cover his mouth with horror.
"Ma?" said Arjun, striding forward to put a protective arm around Kunti and glaring daggers at Karna. "Is he bothering you? We knew he was up to something, intruding upon our camp every night-is he trying to manipulate you into something?"
Karna's eyes flitted from Arjun's flushed face to Kunti's bloodless one.
"Because there is no reason you should put up with this," spat Arjun. "You are coming with us to our camp right now, Ma, I don't care you're officially from Hastinapur, and if he puts a foot into it again, I will-"
Arjun's anger seemed to have snatched away his curiosity for his original question and given them a good enough opportunity to shirk around the truth. 
Now that Karna had thought it through, he did not want Arjun to know. It would not be a fair competition if he was told: it would not be a competition at all, and he did want a competition so badly. Arjun fighting with all his heart was the essential step to all of Karna's dreams.
"By all means, take your mother to your camp, Arjun," said Karna coldly. "That's where she should be."
He marched up the road to the Kaurava camp.
It did not matter that much, after all, what Kunti Ma had told him. He hated Arjun so much, it did not even matter if they were related by blood. Karna would kill Arjun with his bow and arrow: and that was final. He would kill him now, if he could, but he did not have his weapons. 
Arrogant, possessive bastard, thought Karna, before catching himself, for the last word was applicable not for Arjun but for him.
"What was he talking about, Ma?" Arjun's voice floated back to him.
So convinced was Karna about his birth mother's intentions of trying to weaken him to improve Arjun's chances, he had not foreseen her reply for one moment.
"He wanted me to tell you that he is your older brother, Arjun."
Karna stopped walking. Even the owls fell silent.
"What?" demanded Arjun. "Why would he want you to tell me something like that?"
"Because it is true," said Kunti.
"What does that mean? He's-he is-your son?"
"Yes. Years before your brother Yudhishthir was born, even before I married your father, he was a boon from Suryadev."
Arjun seemed finally to have been stunned into silence. 
Terrified now that it was out in the open, Karna speeded up, but it wasn't enough to avoid Arjun's voice, which was speaking again.
"That can't be true, Ma. He can't be our brother. He's-he's-the son of-"
"Do not discriminate him for the caste the world believes him to belong to, Arjun," said Kunti.
"I-I just meant-everyone knows his parents, Adhirath and Radha-"
"They are the ones who raised him when I abandoned him."
"Why did you abandon him?" asked Arjun. "Did you foresee he'd turn out like this? If so, I am proud of you."
There was no reason for the anger to be replaced with misery in Karna's heart precisely at that moment, but that's what happened, and he had no answer to the misery.
"No, Arjun, if I had foreseen anything of the future, I would not have abandoned him," said Kunti, who also sounded miserable. "If I had known this was how he would have turned out, I certainly could never have disowned him."
Karna's throat closed up.
"What are you saying, Ma? What are you playing at?" asked Arjun, sounding bewildered and desperate. "Are you ill? Why-why do you suddenly believe Karna is your son? What did they do to you?"
"They did not do anything to-"
Arjun broke into Kunti's words with a commanding shout.
"King of Anga, halt for a moment."