Janamejaya

There is a story about the emergence of Guruvayoor Temple in Narada Purana. King Parikshit, a descendant of Kuru dynasty, being the grandson of Arjuna and son of Abhimanyu, died due to the bite of Takshaka, a fierce snake, after the curse by a sage. His son, Janamejaya, tried to revenge this by conducting a fierce yaga named Sarpasatra. Many innocent snakes got killed in the ritual fire. But Takshaka did not die, because he had drunk Amrita. Thus, Janamejaya got cursed by snakes and he was affected with severe leprosy. Even though he tried his best to get cure, his condition did not improve. Thus, both his body and mind got weaker over time.

At that time, sage Dattatreya appeared before him and requested him to worship Lord Mahavishnu of Guruvayoor to get cure of leprosy. He also told about the greatness of the idol there. That story is like this:

Long ago, in Padma Kalpa, when Lord Brahma was conducting his task of creation, Lord Vishnu appeared before him. When Lord Brahma requested his wish to get salvation to himself and his creations,

1. Lord Vishnu gave an idol of & made by himself. Later, in Varaha Kalpa, 
2. Lord Brahma gave this idol to a king named Sutapass and his wife Prasni, who were worshipping Lord Vishnu to get a son. They continued to worship, and finally Lord Vishnu appeared before them. 

He stated that he would himself be born as their son in four births, and in all those births, they would be blessed with the idol which they worshipped. 

1. Thus, in the first birth in Satya Yuga, the Lord was born as Prasnigarbha, the son of Sutapass and Prasni. 
2. Later, in Treta Yuga, when Sutapass and Prasni were born as Kashyapa and Aditi respectively, the Lord was born as Vamana, their son. 
3. Again later, in Treta Yuga itself, when they were born as Dasaratha and Kausalya respectively, the Lord was born as Rama, their son, and 
4. finally, in Dwapara Yuga, when they were born as Vasudeva and Devaki, the Lord was born as Krishna, their son. In all these births, the idol was also with them. Later, Lord Krishna, an avatar himself, took the idol to Dwaraka, and began to worship it.

At last, when the Lord was ascending to heaven after his avatar, he told to his friend and devotee Uddhava that Dwaraka would be submerged with sea within a week and everyone expect the idol he worshipped would be destroyed, so that the idol should be given to

1. Brihaspati, the guru of devas and 
2. Vayu, the wind god. 

What Lord Krishna told exactly happened. Uddhava took the idol from the sea and gave it to Brihaspati and Vayu. Later, he went to Badaryashram to do penance. Brihaspati and Vayu went southwards with the idol. On the way, they saw a large, beautiful lake very close to the sea, which was full of lotuses of all colours. They entered there. They saw a huge forest near the lake. Everyone there was so exciting that even Indraloka would be defeated. Many birds were chirping, animals were walking through and fresh air was all there. When they walked through the place, they met with

1. Sage Parasurama, who took them to the place where his guru 
2. Lord Shiva and 
3. Goddess Parvati were sitting. All the three told about the greatness of the place they were standing. It is like this:

Long ago, Lord Shiva worshipped Lord Vishnu in the place which they were standing. The lake on the north side is known as Rudratheertham, as Rudra (Lord Shiva) bathed there before conducting poojas. Many years later, ten prince brothers named 'Prachetas' came to the place to get the title of Prajapati. Lord Shiva taught them a hymn named 'Rudrageetham', in praise of Lord Vishnu. They meditated with that hymn, and finally their wish was fullfiled.

When Brihaspati and Vayu heard the story, they realized that the place itself is suitable for consecrating the idol. Thus, they called Vishwakarma, the architect of devas. He built a temple within minutes and the idol was consecrated there. As it was Guru and Vayu who consecrated the idol of Lord Vishnu there, the place came to be known as Guruvayoor and the idol came to be known as "Guruvayoorappan".

http://paninformation.blogspot.com/2011/09/janamejaya-two-versions-place-festival.html