It was Drona. Drona was married to Kripa's sister but the Pandavas had no idea who he was, not even the fact that Drona was the greatest warrior in the world, having gotten his learning from Parasurama himself.
``I shall bring up your ball with a blade of grass and the ring with an arrow,'' he told the Pandavas, ``if you will promise me a dinner tonight.''
``You ask for very little,'' replied Yudhishthira, ``we promise.''
Drona took a blade of grass and pierced the ball with the sharp end of the blade. He threw in a second blade which stuck to the base of the first and so, blade after blade made a long chain. Finally, as the Pandavas watched in amazement, Drona casually yanked the ball out of the hole using the chain of grass blades.
``And now the arrow,'' said Drona with a smile. He drew an arrow out of his quiver and released it into the hole. The arrow pierced the ring and when it bounced back of the hole, the ring was attached to its head.
Drona returned the ring to Yudhishthira. ``Now for dinner,'' he said to the awestruck princes. The Pandavas took him with them to the palace.
Bhishma heard of the skill of the brahmana and knew that this could be none other than Drona. ``Welcome to Hastinapura,'' he said, ``it is our good fortune that you are come here.''
``I am here,'' said Drona, ``because Drupada refused to pay me due respect. .''
``It is his loss,'' said Bhishma, ``Drupada is a fool.''
``Won't you stay,'' Bhishma asked Drona, ``and teach, with Kripa, these young princes and your own son, your military skills?''
Drona agreed to teach them all. Drona of course hoped that his son, Aswattama, would excel the royal princes but it was Arjuna who learnt the most from his teacher.
When the lessons were finished, Drona called the princes all together and told them that as repayment for their Guru's lessons, they should attack and bring back the Panchala king, Drupada. Arjuna captured the king and brought him back to Drona, who finally had his revenge on the Panchala king .
Once Drona's revenge had been exacted, he decided to hold an exhibition of his students for the benefit of King Dhritarashtra. The Pandavas did well, so did Duryodhana, but Arjuna outshone them all. Yet, by the end of the day, it was Karna, half-brother to the Pandavas but son of a charioteer , that emerged the best warrior of them all .