Makardhwaja

After burning down the whole of Lanka with his inflamed tail, Hanumanji took a dip in the sea. Just then a drop of his perspiration fell into the mouth of a giant fish-eat-reptile like creature, Makara, thus impregnating it.

The fish was caught by people of Ahiravana, who ruled Patala, the nether-world. Markardhwaja was discovered when the stomach of the fish was cut open and thus he was named after it and was brought up by them! 

When he grew up, Ahiravana, seeing his strength and virility gave him job of guarding the gates of his kingdom.

When Ahiravana took Rama and Lakshmana to Patala, Hanumanji followed them in order to rescue Rama and Lakshmana. He was challenged at the gate of Patala by a creature, who was part Vanara and part reptile - a Makara. He introduced himself as Makaradhwaja and son of Hanuman, the mighty warrior.

Hanumanji was amused and said that he himself was Hanuman but Makardhwaja could not be his son, as he was Poorna Brahmachari/lifelong celibate since birth. However, Hanumanji then closed his eyes in dhyana to see the events related to Makardhwaja's birth.

Makardhwaja asked him for his blessings. However, he said to Hanumanji that he will have to fight him in order to enter Patala, as he cannot betray Ahiravana, his swami. Hanumanji defeats Makaradhwaja in a duel and then ties him to the back and proceeds further to ultimately kill Ahiravana and rescue Rama & Lakshamana.

Makardhawaja is presented as the exemplary devotee of Ahiravana just like Hanumanji was devoted to Lord Rama. While returning from patal lok Shri Rama looked at Makradhwaj in pain and asked Hanuman Ji about him. Hanuman ji narrates everything to Shri Ram. Shri Ram was very impressed by Makardhwaj's devotion for his mentor and his fight with Hanumanji . After that He asks Hanumanji to release Makardhwaj and declares the later as the king of Patal Lok since the then king , Ahiravan was killed.