There are many subtleties and esoteric meanings connected with the mythology and legends of Arunachala. One of the most famous being that of Ardhanarishvara - a form of the divine which is particularly celebrated at Arunachala and not more so than during the festival of Deepam and on the day of Bharani Deepam.
At the mystic hour of dusk (pradosham), when thousands have gathered in the courtyards and roofs of the Temple, waiting since early morning, the pancha murtis, are carried out into the courtyard in a fast running motion, sitting in their golden palanquins covered with festoons. The deities are placed in a Mandapam (ceremonial pavilion) opposite the entrance to the Arunachaleshwara's temple and facing the holy mountain.
At that time also the deity Ardhanarishvara is brought out and placed on the stairs of the temple close to the big Deepam. This is the only day of the year that this particular Deity is ever moved. It is most auspicious.
In the Deepam Festival is also reflected the union of Shiva and Parvati in the deity Ardhanarishvara. Once the goddess in play covered the eyes of her Lord Shiva with her hands, and thus the whole world was plunged into darkness. However, Shiva opened his third eye on the request of the gods, and the light was restored.
Uma was ashamed of her childish behaviour, and she retired from Mount Kailasa to Kanchipuram to do penance and purge herself of her sin. Shiva then directed her to go to Tiruvannamalai to worship him there. Mother Uma became an anchorite and did hard penance, going around Arunachala hill with deep concentration on the holy name of the Lord.
Shiva was pleased with her, and told her that she was now relieved of her sin which was causing the untimely pralaya (destruction of the world). He blessed her and said, 'Come and unite with me,' and disappeared in the hill.
Then on Kartikeya day the Lord appeared as a blazing light, a jyoti on the top of the hill, and asked Mother Uma to circumambulate the hill. So she did, and when she rounded the western side of the hill, Shiva appeared on his white bull and blessed her. When she rounded the hill on the north-western side he absorbed her into the left half of his body. Thus came into being the form of Ardhanarishvara, the deity that is represented as half male and half female.
To learn more about the 'Legend of Ardhanarishvara' at Arunachala go to this link here:
http://arunachalagrace.blogspot.com/2007/12/ardhanarishvara-at-arunachala.html