Indra blessed him with an astra called Vashavi shakti which could be used once on any enemy who was facing him in a battle and after slaying his enemy the astra would return to Indra.
Lord of universe Shri Krishna being aware of the weapon kept Arjun's chariot away from Karna for the time being
On the 14th day of battle, After the death of Jaydrath, the war didn't halted after sunset and the Pandava army was on the verge of losing. That's when Lord Krishna asked Ghatotkach to take his Asura form and destroy Kaurava's army, which was against the rule.
