Raudreya is one among the numerous names of Lord Hanuman. There are numerous stories in Hindu tradition, which narrates the birth of Hanuman. Some stories are of the view that Hanuman is son of Shiva, some claim he is an avatar or incarnation of Shiva, some say he is amsha or manifestation of Shiva. Raudreya is used to refer to Hanuman when his birth is associated with Shiva.
Legend has it that Vishnu took the form of Mohini to
distribute the Amrut, or elixir of life, that the Devas and Asuras received
after the Samudra Manthan or Churning of ocean. Asuras and Devas started a
fight over who will have the Amrut first. Mohini appeared to distribute the
Amrut peacefully.
Shiva who happened to know about the beauty of Mohini wanted
to see it. Shiva was enamoured by the beauty of Mohini. Scintillating nature
and the beauty of Mohini made Shiva spill his semen. The semen spilled was
collected by Rishis in a leaf and deposited in the womb of Anjana – who was
meditating on Shiva for a son. The modern name of this technique is Artificial
insemination.
Please note the popular story of birth of Hanuman is that He
is the son of Vayu and Anjana. Anjana who did not have a child prayed to Vayu –
the wind God. He blessed her with Hanuman.