There are some stories from Ramayana which might not have been written or talked about by people in either the school literatures or the smaller version of holy books that you read everyday.
Ahiravan, the brother of the demon king Raavan, was the ruler of Paatal Lok and a master of trickery and black magic just like raavan’s son Indrajit but not that powerful. Apparently when Indrajit died during the war with the Vaanar sena by the hands of Lakshman, King Raavan summoned his brother for help before entering the war himself. Ahiravan promised to capture Ram and Lakshman alive, take them to his kingdom, where he would sacrifice them to Mahamaya, the Goddess of Illusions. Vibhikshan on learning about the plan of Ahiravan, warned Ram and Lakshman about the danger that Ahiravan could pose to them and asked Hanuman to not to trust anyone while guarding the tent where Ram and lakshman slept.
Ahiravan came at night and saw Hanuman guarding the tent, and he knew that until Hanuman was guarding the tent, he won’t be able to achieve his objective. So he disguised himself as Vibhikshan, and somehow convinced Hanuman that it was him –original Vibhikshan and Hanuman fell prey to Abhiravan’s disguise. He entered the tent of Ram and Lakshman, and in their sleep took them to the Paatal Lok.
Next Morning, when Hanuman realized that he was tricked by Ahiravan and that Ram and Lakshman’s life was in grave danger, he pledged in front of Vibhikshan that he would bring them back safe and sound and kill Ahiravan.
Hanuman goes down into the Paatal Lok where he finds the palace of King Ahiravan. He saw that it was heavily guarded and he was to contend with the guard, Makardhwaja, who is half Vanara, half fish, and also happens to be the son of Hanuman in a way. After defeating his son, Hanuman enters the palace where Ahiravan was waiting for him. Ahiravan was laughing at Hanuman and mocking him that he could not be killed in his own kingdom.
Suddenly, Hanuman realized that there were 5 strange looking lamps kept in different directions of the Kingdom. Ahiravan could be killed only when all of these lamps were extinguished at the same time.
This is when Hanuman assumed ,what is called in holy books as Panchamukha Anjaneya form. The 4 faces of Hanuman: Varaha, Garuda and Narsimha faced east, south, west and North respectively, while the fifth face called Hayagriva faced upwards. He extinguished all the lamps and when Ahiravan was weakened, Hanuman killed him with one single swift stroke of knife.
That’s how Hanuman saved lives of Ram and Lakshman and brought them back where on the next day of war, Ram defeated and killed Raavan.