Karna gives away his armor

Bhima was devastated to see his son die. Taking his mace, he began killing soldiers, horses and elephants. The soldiers were tired, having fought all night but fearing Duryodhana they could not refuse, so agreed to face Bhima and die.
Arjuna asked for the war to be called for. All were tired and needed rest.
He then sat with Bhima trying to console him. Arjuna said I understand your pain. I too lost Abhimanyu. Krishna, Please tell us what we should do now.
Krishna said Know that death of Ghatotkacha means life for you, Arjuna.
Karna was born with armor and earrings. With these, no weapon could pierce him.
Also, Karna was a great believer in charity and never refused anyone who came to him.
Knowing this, Indra went to Karna in the guise of a brahman.
Karna saw the brahman and bowed down to him and asked who he was and where he was coming from.
The brahman said he had come from far for alms.
Karna said What do you want.
The brahman said First vow that you will give me what I ask for.
Karna wondered who this was in the guise of a brahman. Leaving the thought aside, Karna promised that whatever was asked for, would be given.
The brahman said Then give me your armor and earrings.
Karna remembering Surya telling him about this. He then said I know you are Indra. You have come for Arjuna. If you had asked for my life, I would have given you that. Why do you fear for Arjuna when Krishna is with him?
Saying this, Karna took a sword and then cut off the armor and the earrings from his body.
Indra said Ask for something.
Karna said I know of a weapon that you have that will never miss its mark. Give me that. Indra gave that to Karna, and left with the armor.

Arjuna, Karna had been saving this to use on you. He had to use it on Ghatotkacha thus saving your life.