Fire or Agni is the fourth element of the Panchabhutas. The essential character of Fire is to generate heat and according to Hindu Mythology, Agni is one of the Eight Guardians that guard our Universe and are known as Ashta Dikpalakas.
As per Indian mythology, there are mentions of various types of fires. The four important ones are the fire of the earth, the fire of the sky, the fire of the stomach and the fire we commonly use.
As for Lord Arunchaleswarar temple, there is a wealth of mythology attached to Arunachala. One such myth is that Lord Surya estranged from his wife Chayadevi visited Arunachaleswarar Temple to propitiate the Lord.
A brief on the mythological story – It concerns Singavaram and a Temple dedicated to Sri Ranganadha. Here a king named Kandhan, after suffering defeat, performed a fire walking ceremony in the worship of Draupadi (from the Mahabharata). Draupadi, pleased with the worship, appeared before the king and promised that whoever should worship her as he had done would remain unscathed and also obtain whatever they prayed for.
From that day onwards the fire-walking festival is observed with great enthusiasm throughout Tamil Nadu including Thiruvannamalai temple.