Sarg 66 - Lav-Kush are Born 

When Shatrughn was sleeping in Vaalmeeki's aashram, some Muni children told Vaalmeeki about midnight that Seetaa had given birth to two sons. Vaalmeeki Jee came there and he guarded them with "Bhoot Vinaashinee" Mantra. The child who was born from the front part of Kush, was thus named Kush and child who was born from the back part of Kush was named Lav. Then Muni women sang some auspicious songs.

Bharat also heard the sweet sound of their singing so he also arrived there and said - "Devee, It is very fortunate that two sons are born." In the morning he went to west side after greeting Muni. On 8th day he arrived in Chyavan Muni's Aashram. There Shatrughn talked to many Muni and passed that night happily.

Sarg 67 - Chyavan Rishi Tells Lavanaasur's Powers 

Next day he asked Chyavan Rishi - "Hey Bhagvan. Tell me something about the powers of the trident  Lavanaasur has. How many people have been killed from that trident?" Rishi said - "Oh dear, Lavanaasur has troubled a lot. Long before, there was a king of Ayodhyaa, named Maandhaataa in your Ikshwaaku Kul. In a short time Maandhaataa had won the Prithvi. Indra etc all Devtaa got scared to see this. Indra has already given His half kingdom to Maandhaataa, still Maandhaataa wanted to have the whole Swarg. He took the vow for that. 

Indra tried to explain him - "If you wish to rule the Swarg that is very unjust. First you should have control over the whole Mrityu Lok, after that you may come here and rule it." Maandhaataa asked - "Indra, Which place on Prithvi is not mine, tell me?" Indra said - "Lavanaasur of Mathuraa does not obey you." Hearing this Raajaa came on Prithvi feeling embarrassed, and headed to fight with Lavanaasur with his army.

Raajaa sent a messenger first, but when he did not return, then he started shooting arrows, but his all arrows failed and the Raajaa was killed by Lavanaasur's trident. So Hey Shatrughn, Lavanaasur's trident has lots of powers. The best time to catch him is in the morning when he goes out to bring meat without carrying his trident. Kill him when he comes back. May Bhagvaan bless you, you will surely win."

Sarg 68 - Lavanaasur Finds Shatrughn at His Door 

Shatrughn did as Rishi advised to him. When Lavanaasur had gone in the search of meat, Shatrughn went and sat at his door. Lavanaasur came back in the afternoon carrying many living beings, he found Shatrughn at his door. He said - "Hey low-level man. Certainly Bhagvaan has sent you as my food. Now you cannot escape." 

Shatrughn said - "O evil, I am brother of Raam and I am not going to leave you alive today. Come and have a dual fight with me." Lavanaasur said smiling - "Raavan was my Maamaa. Raam has killed his whole Vansh, but I did not say anything. But today you are insulting me in front of me? This is just intolerable. Just wait, let me bring my weapon, then I will fight with you." Shatrughn said - "It is not good to leave the enemy after getting hold of him. Therefore you will not be able to go from here and will be killed by me."