Wikipedia
In the Hindu epic Ramayana, Lavanasura was an Asura (demon) who was killed by Shatrughna, the youngest brother of Lord Rama.
During Rama's reign, while peace prevailed in most places, Lavanasura continued to torment the innocent and destroy many sacrifices of sages and terrify them in many ways. Many kings were defeated by him and they were all afraid. Therefore, one day the sages headed by sage Chavana (a descendent of sage Bhrigu) came all the way from Madhuvan to Lord Rama with a plea to protect them. Lavana was the son of king of Asuras named Madhu. Madhu was kind and compassionate to Brahmanas, very exalted, made personal friendships with Devas and therefore Asuras and Suras were at peace. He was so pleasing to Gods that on one occasion, Lord Shiva gave him an expansion of his own trident for self-protection. Madhu built a palace and named the place as Madhupuri (possibly Mathura now). Madhu had a son named Lavana who had the qualities opposite to those of his father.
Lavana was so evil that even as a child he would beat play mates, kill them, and eat them. Madhu handed everything over to his son including his trident and drowned himself in the ocean because of shame. The sages further described Lavana. There was a king named Mandhata (or Mandatta) who was a descendant in the dynasty of Iksvaku. Mandhata had dominated the whole planet and he became so proud that he wanted to rule heaven also. So he challenged Indra-either to turn over the kingdom to him or fight with him in battle. Indra said, "If you defeat all personalities on earth, then I will give you my kingdom." Lavanasura defeated Mandhata's army with the trident. Hearing all this, Lord Rama promised He would protect the sages and the kingdom of Madhupuri.
Bharata volunteered to fight against Lavanasura. Shatrughna expressed his wish to fight Lavanasura, wanting an opportunity to serve Rama, stating that Bharata had served Rama very well in the past. Then, all brothers agreed that Shatrughna could go. Shatrughna, apologising for violating the etiquette of speaking against the wish of his elder brother Bharata, then prepared to leave for the war. Rama then gave Shatrughna a deity of Varaha, which manifested from Lord Kapila's mind. When Lord Rama defeated Ravana, this deity was taken to Ayodhya. Shatrughna was then crowned as the king of Madhupuri. Shatrughna then left with a huge army to fight with Lavanasura. Rama warned Shatrughna to find a way to fight with Lavana without his holding the invincible trident. Shatrughna stood before the gate where Lavana resided and positioned himself alone. As Lavana returned home after hunting animals for eating each day, Shatrughna challenged him to fight. Lavana was very happy to accept the challenge because it was his dessert time. Lavana uprooted many trees and threw them on Shatrughna, and a big battle ensued. Later, Shatrughna removed the special arrow (used for killing Madhu and Kaitabha, given as a gift by Lord Vishnu) that Rama had given him. As Shatrughna strung his bow, the whole universe started to tremble. He struck Lavana right in the heart taking out his life-breath. Rama then installed Shatrughna as the king of Madhupuri, where he ruled for several years.