Who is the Son of Lord Hanuman?

The son of Lord Hanuman is named Makardhwaja (मकरध्वज). He was a mighty warrior who was half-Vanara (monkey) and half-Makar (a mythical fish/crocodile).

Since Hanuman was a celibate, Makardhwaja was not born through normal means.

According to the legend, after Hanuman burned down the city of Lanka (Lanka Dahan) with his flaming tail, he took a dip in the ocean to cool off and soothe his burns. Due to the intense heat and exhaustion, a drop of his sweat fell into the ocean.

A giant fish (or reptile) named Makardhwaja swallowed this drop of sweat. From this divine sweat, the fish became pregnant and later gave birth to a powerful son. The son was named Makardhwaja, after his mother.

Makardhwaja grew up to become the guard of Patal Loka (the underworld), which was ruled by Ahiravan (Ravana's brother).

When Ahiravan kidnapped Lord Rama and Lakshmana and took them to Patal Loka, Hanuman went down to rescue them. There, he was stopped at the gates by a powerful warrior who introduced himself as 'Makardhwaja, the son of Hanuman'. After a fierce but respectful battle, Hanuman defeated his son, rescued Rama, and later crowned Makardhwaja as the new King of Patal Loka.