Dronacharya was a Guru of Rajputra, means he was appointed by the King Dhritrashtra to teach the sons of Pandavas and Kaurvas. Secondly, there was then rule that except Kshatriye no other had the right to learn archery knowledge. Because, the whole and sole responsibility of the society was to be checked or safeguarded by Kshatriye King and his sons.
Ekalavya was the son of a Saarthi, who used to play the chariot of King Jarasandh of Magadh (Bihar). Eklavya had no right to learn archery, but he approached Guru Dronacharya for learning archery. Guru Drona rejected his proposal and told him to keep away from archery.
So, being distress and desperate, Eklavya thought to make a idol of Guru Drona and started learning lonely at his hermitage Dhanu Vidhya/ archery. Hence, he began to learn archery before his guru idol Drona.
One Day, Guru Drona along with his pupils like Pandavas and Duryodhan were being taught in the open field. Suddenly, a barking dog was moving near Eklavya, so he bounded the mouth of Dog by the arrow. The barking Dog stopped barking. Suddenly, the Pandavas looked the Dog filled up its mouth by arrow. So, to know, who was behind the arrow, Guru Drona sent his pupil to check.? Pandavs and Druyodhan were surprised to see the skill of the Archer.
When Guru knew that it was Eklavya was learning archery before his idol, he was utmost surprised to see it. So, he called out Eklavya and asked his Guru Dakshina? Drona said that Eklavya had made him fool, because, he learnt the archery without any knowledge to him. So, he must pay ‘Guru Dakshina’
So, Guru Drona asked his right hand thumb to him in his Guru Dakshina. Eklavya knew well that if he would donate his right thumb, he could not be able to shoot arrow with his bow. But he was happy and donated his right thumb to Guru Drona. Eklavya told Guru Drona that he was his Guru and he would continue to bow his head before his Guru.
Even after donating his right thumb, he became skill in archery. He was then elected as the most skill archer in the army of Magadh Naresh, Jarasandh. Jarasandh was a great devotee of Lord Shiva and he was considered the most mighty king in whole India. Eklavya father was already there in the army as the Saarthi of Chariot, so Eklavya began to work for Jarasandh, Magadh Naresh.
So, the fight erupted between Sri Krishna and Kansh. Kansh was the son-in law of Magadh Naresh Jarasandh, when he learnt that Kansh was killed by Sri Krishna, he began to be jealous and developed hatred against Lord Sri Krishna. He made so many plans to kill Lord Sri Krishna, but he failed in his mission.
Thus, Jarasandh invaded Mathura 18 times to defeat Sri Krishna. Finally, knowing the will of Jarasnadh, Lord Sri Krishna shifted to Dwarka with his all subjects. He killed Shishpal, who was the close friend of Jarasandh. Later, Lord Sri Krishna became the King of Dwarka as Dwarkadhish.
So, when Sri Krishna along with Arjun and Bheem went to Magadh(Bihar) to challenge Shiv Bhakta Jarasandh to fight in open field. Being the great devotee of Lord Shiva, he used to fulfill the wishes of the people, if anyone asks/ In MalYuddh, it was not easy task for Bheem to defeat Jarasandh, but due to the trick played by Lord Sri Krishna, Jarasandh was Killed and Yudhishter was crowned the Naresh of Bharatvarsha.
So, knowing all the tricks of Lord Sri Krishna, Jarasandh was killed, so being devoted servant of Jarasandh, He along with his mighty army invaded Dwarka and he tried to defeat Sri Krishna, but he was first defeated by Balram Ji. Eklavya fought fiercely with Yadvas and defeated the Yadvas army, but finally, Sri Krishna came in the battle field and he was killed by him.
But during the time of death, being Bhaktvatsal, Lord Sri Krishna reached there and asked his will if any? Eklavya told his whole story to Lord Sri Krishna and about the tricks and cheating by Guru Drona. Lord Sri Krishna granted him boon that he would rebirth as Drhishtdhumn as the son of Panchal Naresh(Punjab).
Finally, Eklavya died and he rebirth in Kshatriya dynasty as the son of Panchal Naresh Dhrishtdhumn. You must be knowing that Dronacharya was killed by Dhrishtdhumn in Mahabharata war. It was Lord Sri krishna, who instigated Drishtdhumn to cut off the head of Guru Drona. Thus, this is the whole story of Guru Dronacharya and Eklavya