Aalingan

Vishnu appeared as Mohini twice.

The First Time "She" appeared was during Sagar Manthan.

Vishnu appeared in His female avatara Mohinimurti, a hypnotically beautiful form that captured the minds of the demi-gods and demons alike. While distributing the Amrita (nectar of immortality) Mohini gave it all to the Devas (gods) and thus cheated the Asuras (demons). However, Rahu being very intelligent snuck between two Devas--the Sun and Moon gods and was able to sip some of the Amrita. But before he could swallow it he was found out by Surya and Chandra who announced his presence. At this point Mohini Murti used Her transcendental waepon the Sudarshana Chakra to cut off Rahu's head. But because he had touched the Amrita to his lips his head stayed alive while his body died. That is why the planet Rahu is described as having no body but only a head.

After cheating the demons (asuras), Mohini moved around for sometime. During this time Shiva, who had heard a lot about "Her" beauty wanted to embrace "Her". However, Mohini smiled and said that when the time would come "She" would allow Him to do so. Saying this, the Mohinimurty of Vishnu dissapeared.

However, as Time would have it, Vishnu did have to appear as Mohini the second time.

Here goes the story:

Bhasmasura was a wicked demon. He wanted to be a powerful king. But he was neither strong nor brave. How could he defeat other kings?One day he met the great sage Narada while wandering through a forest, and said to him: "O great sage, tell me how I can become great and strong-so strong that I may be able to defeat even the greatest kings.""Worship Lord Shiva," replied Narada, "and you will get magic powers from him."Bhasmasura went to a thick jungle. There, finding a quite place under a pipal tree, he settled down and began to worship Shiva. Many years passed before his prayer was heard by Lord Shiva.At last, pleased with his devotion and prayer, Lord Shiva appeared before him, and said: "I am pleased with you, O Bhasmasura. Ask a boon, and I will grant it.""O Lord!" said Bhasmasura, "I only want your blessings. There is no other purpose behind my devotion to you.""You have my blessings," said Shiva. "Still, would you not like to have a boon from me?""Well, my Lord," replied Bhasmasura, "if you are really pleased with your humble servant, grant me the power that any person whose head I may touch with my right hand will at once die and be reduced to ashes.""Be it so," said Lord Shiva. "Anybody whom you will touch with your right hand will die and be reduced to ashes."

Armed with such magic powers, the wicked Bhasmasura jumped up towards Lord Shiva to touch his head with his right hand. By destroying Shiva himself he dreamed of becoming the King of Kailas!Shiva's life was now in danger. He knew that he would be reduced to ashes if Bhasmasura was allowed to touch his head. So he ran for his life.Bhasmasura ran after Shiva. Shiva ran over hills and mountains as fast as he could, but Bhasmasura was always close at his heels, with his right hand raised, ready to touch Shiva's head and reduce him to ashes.Shiva wanted somebody to come to his help immediately and save his life, since Bhasmasura was sure to kill him. So he prayed to Lord Vishnu to save his life.

Vishnu heard Shiva's prayer and changed himself into Mohini, a maiden of wondrous beauty. She looked far more beautiful than any goddess in heaven. When she danced in the moonlight, Bhasmasura stood rooted to the ground struck with her beauty."Who are you, beautiful maiden?" he asked. "And where did you spring from?"She replied shyly: "I am Mohini. I live in this jungle with my parents. We always help those who pass this way. You look very tired and hungry. May I bring you some fruits to eat?"

"No, don't go away from me, beautiful maiden," replied Bhasmasura. "I have never before seen any maiden so beautiful as you are. You are far more beautiful than any goddess. Tell, heavenly nymph, will you be my bride? I have lost my heart to you.""I would love to marry someone as strong and mightly as you," replied Mohini, "but I have one vow to fulfill before I become your wife.""And what's that?" asked the demon."It is a very simple vow," said Mohini, smiling. "When I was child I took a vow that I would marry only that person who would dance with me on my wedding night. Will you dance with me?""Certainly," replied Bhasmasura, "I am prepared to do anything for you. It will give me the greatest pleasure to dance with a beautiful maiden like you."Mohini laughed and said to Bhasmasura "Now watch me closely and do exactly what I do. The man who wishes to marry me must dance with me, copying all my movements."

Mohini started dancing. First she put her hands on her hips and Bhasmasura did the same. Then she raised her hands to heaven and Bhasmasura copied her. Finally, she put her hand on her head. And Bhasmasura, whose eyes were constantly fixed upon Mohini's beautiful face, completely forgot about Lord Shiva's boon, and put his hand on his head too.There was a loud roar of thunder and the huge demon crumbled to ashes at her feet!What a relief it was to Lord Shiva! He at once appeared before Mohini and thanked her a lot for saving his life. The two embraced each other.

It was this time that Shiva embraced Vishnu in the form of Mohini and the union thereby resulted in the birth legends of Hanuman and Ayyappa.