Jayadhratha learns about Arjuna's vow from his spies and becomes afraid and does not know how to face this threat to his life. He meets the kings in the kaurava camp and laments, 'I heard that Arjuna has made a vow to kill me tomorrow. Even devas and gandharvas cannot stop him, how can I stand before him alone? If you cannot assure me of my safety, I will quietly go away from here and hide in my country. Give me permission to leave.'
Duryodhana tries to bolster the confidence of his brother-in -law, 'Would Drona, Karna, Dussasana and many more heroic warriors forsake you? You are also no less in your courage and abilities.' He then, in the very night, takes Saindhava to meet Dronacharya.
Drona promises him full protection.'I will construct an impregnable formation which will be impossible for Arjuna to penetrate. Do not be scared of Arjuna. Fight like a true warrior!' And consoles him with, 'We are all in line for death and it is inevitable. Let us do our duty and be ready to go to heaven.'
In the morning, Drona forms a शकट(wedge) vyuha and behind it a Padma vyuha and inside it a सूचि formation. Creates the sharpest offence with Krithavarma, Kamboja, Duryodhana, Karna and many more in front and places Jayadratha behind them. Drona places himself in front of the shakata vyuha.
The war begins, Arjuna defeats Dussasana, Duryodhana in no time and approaches Drona and salutes him. 'Acharaya! Please bless me. You are like my father. You should protect me like you would Ashwatthama. With your loving kindness, I propose to fight with Jayadratha and kill him.'
Drona smiles, 'How can you hope to defeat Jayadratha, without first defeating me?' and starts showering arrows on Arjuna. Krishna realises that there is no way to defeat Drona and suggests that they should not waste time with Drona and suggests, 'Let us leave this Brahmin alone and go further'. Arjuna agrees and Krishna deftly manoeuvres the chariot away from Drona. Drona noticing this shouts 'Hey! How can you leave without defeating your enemy?'
Arjuna replies as they leave Drona, 'You are not my enemy, you are my guru and I am like your son. And there is no one who can hope to defeat you.' And they suddenly turn and penetrate deeper in to the karava army. There is a fierce combat and many are killed. But the warriors who are guarding Jayadratha, keeping a low profile, shield him well.
Krishna sees that it is almost evening and thinks of a ploy. He tells Arjuna, 'I will create an illusion of the sun setting. Jayadratha relieved that he is saved, will raise his head and you must kill him at that moment.' It becomes dark and both the armies thinking that the sun has set, stop fighting and raise their heads to look up! Jayadratha also very happy looks up and Arjuna severs his head with a powerful arrow.
If Jayadratha's head had rolled on to the ground, it would have split Arjuna's head into thousand pieces. But Krishna had told him about this. So Arjuna shoots many more arrows at the severed head and it flies towards Jayadratha's father, Vriddhakshatra, who is offering prayers to the sun at that time and lands on his lap. As his father concludes the prayers and gets up, Jayadratha's head rolls down to the ground and his fathers head splits into smithereens.
Jayadratha's father had earlier thrown a curse that whoever drops his son's head on earth, that persons' head would split at that instant into a thousand pieces. The curse thus became the cause of his own death. If not it would have been Arjuna's head. No sooner Arjuna's vow is fulfilled, the sun appears again and people realise it was Krisna's maya!
This combination of myth and reality in the story is intriguing! God should be on your side, to remind and counteract the various boons given at different times to different people for various reasons. Arjuna was lucky that Krishna was on his side!