Entry of Ghatotkacha is a very smart tactical move by Krishna.
On Kauravas side, Bheesma has been killed. The major threat has been removed. The next threat is Drona and Karna’s Vasava shakti. If Karna uses the Vasava shakti, Arjuna is dead. If Arjuna is dead, Pandavas lose since noone else can stop Drona. Arjuna won’t kill Drona, so Krishna’s main concern is to get rid of Karna’s threat and hence, Ghatotkacha is deployed.
Ghatotkacha breaks another rule of the war – surprise attack after sunset.
Here is the conversation between Duryodhana and Drona
What the hell is this? How can the Pandavas attack at night?
Looks like there is a daemon out there. I don’t see any army.
Who is this warrior? He is uprooting my army like grass.
He is ghatotkacha, the asura son of Bheema
We must get rid of him as soon as possible.
This rakshasa can only be pacified with a divine weapon (Divyastra).
Karna, fire your weapon and kill him.
I can’t. I have reserved it for Arjuna.
Now Drona and Ghatotkacha enter a duel. Ghatotkacha severely injures Drona and Duryodhana asks him to use Divyastra.
Drona then tells Duryodhana
“I can use Divyastra against him, but as long as Arjuna is there, I am withholding my Divyastra. If I use my Divyastra against Ghatotkacha, I will not be able to stop Arjuna’s attack. I must not use my weapons now. If I do, this threat is removed but then Arjuna is a bigger threat whom I won’t be able to stop. I am the commander in chief of this army and it is my duty to ensure that this army is safe from Arjuna’s attack.”
You won’t kill Arjuna because he is your student. But you wont use your weapons when its needed because your student is a threat to you. What do I do now?
Then Duryodhana goes to Karna and says –
You have to use your Vasava shakti and save my army. Otherwise, this army is finished today itself and then you can find Arjuna and kill him to prove your superiority”
Karna uses his Vasava shakti and kills Ghatotkacha.
Bheema says to Krishna –
My son is dead and you are laughing? What fun does this give to you?”
Krishna says –
I am not laughing on your son’s death. I am laughing because now Arjuna is safe. From the day this war has started, I have been keeping Parth away from Karna as far as it is possible. Now I can relax a bit”