He had a son born from a Makara. When Rama and Lakshmana were kidnapped by Airavan and Mairavan, Hanuman reaches to Patala to rescue them. He sees Makaradhwaja. Hanuman introduces himself as Rama's messenger and inquires who he was. To this question, he replies as follows:
When Hanuman burnt Lanka with his tail and extinguished the fire with the help of ocean water, he spat phlegm in his throat caused by the smoke. That was consumed by a Makara present in the ocean. From that, I was born.
The Lord Sun was the guru ( Teacher ) of Lord Hanuman in his childhood. Since The Sun can not stop for to teach lessons to Hanuman, Hanuman increase his body such that during whole day when Sun rises to Sun-set his face stairs to Sun. This was Sun has teach to Hanuman. Now one day when Sun rays where too hot, because of this Hanuman got sweat. This Sweat drop went in to a sea and it is drunk by a fish. Later on this fish has given birth who is better known as Makardhwaj. Makardhwaj was living below the land surface ( a place very well known as patal ).
On finishing the study from Lord Sun, when Sun asked Hanuman to get marry with his daughter that time Hanuman told him that he can't marry her as she is like her cousin sister since she is teacher's daughter.
The Lord Hanuman was the avatar of Lord Shiva. The Lord Hanuman was never got married in his whole life. His only aim was to serve the Lord Vishnu's avatar named Ram.
When Hanumn was in Sri Lanka, Ravana ordered the Rakshasas to wrap Hanuman's tail with cloths soaked in oil and ghee. Hanuman extended his tail to such length and size that all the cloths in Lanka would not cover it. Then he reduced his tail of his own accord. The Rakshasas wrapped the tail with cloths soaked in oil and ghee and lighted the cloths. Hanuman expanded his body to an enormous size and began to jump from place to place. The whole of Lanka caught on fire. All the palatial buildings were burnt down to ashes.
Hanuman then jumped into the sea in order to cool and refresh himself. A drop of his perspiration fell into the mouth of a great fish (mermaid) which gave birth to a mighty hero named Makara Dhvaja. Makara Dhvaja is considered the son of Hanuman.
Yes, Hanuman did have a son called Makardhwaja. The story goes like this, after setting Lanka on fire after meeting Sita, he dipped into a river to cool down. During cleansing, Hanuman's seed fell into the water and was consumed by a reptile named Makardhwaja.
Ahiravana, brother of Ravana and king of the nether world, in search on Hanuman came across this reptile and killed it. During the process, they found a child inside the reptile and called it Makardhwaja as well. On witnessing his strength and skill they appointed him as the guard of the nether world.
Later, when Hanuman was trying to save Rama and Lakshmana from Ahiravana. He faced Makardhwaja in front of the gates of the nether world. In order to save his master, Hanuman defeated his own son in battle.
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