Arūṇācala is the foremost kshetra (sacred place) of Bhagavān Śiva. It is the place where the infinite pillar of fire, the jyotir-liṅga, appeared when Brahmā and Bhagavān Vishṇu quarrelled. The hill is that pillar of light, made into a lasting presence on earth.

Bhagavān Śiva himself declared Arūṇācala to be his supreme kshetra. The Vidyeśvara Saṃhitā records what he said: that death here gives moksha (liberation), that any act done here bears fruit a crore times over, and that the very sight of the hill destroys all pāpa (sin).

Origin of the name

Arūṇācala means “the red mountain, the colour of dawn”. Aruṇa is reddish, the colour of dawn or fire, and acala is a mountain, “the one that does not move”.

The hill has this name because the endless pillar of Bhagavān Śiva rose from this place, blazing like a mountain of fire (agni-parvata). At its base, where it came out of the earth, it was rosy and bright. The hill is the settled, earthly form of that endless pillar of fire, which once blazed between the higher worlds and the nether worlds.

The Vidyeśvara Saṃhitā (Chapters 7–9) tells this. The full account of the Liṅgodbhava, the appearance of Bhagavān Śiva as the endless pillar to end the quarrel between Brahmā and Bhagavān Vishṇu, is told on its own page.

Bhagavān Śiva declares Arūṇācala supreme

At the end of the Liṅgodbhava, Bhagavān Śiva appeared in his Śaṃkara form before Brahmā and Bhagavān Vishṇu. He then spoke of this kshetra. These words are his own, kept in the Vidyeśvara Saṃhitā (Chapters 8–9).

Moksha. Whoever dies at Arūṇācala gains moksha, freedom from the cycle of birth and death. This holds whatever the manner of death or the state of the person’s sādhanā (practice). To leave the body at Arūṇācala removes the karma that would otherwise bring a new birth.
Phala a crore times over. Any dharmic act, pūjā or rite done at Arūṇācala gives a crore (ten million) times the usual fruit (phala). One pradakshiṇā (walk around the hill) equals a crore pradakshiṇās of an ordinary liṅga. One lamp offered here gives the fruit of a crore lamps elsewhere.
Destruction of pāpa. The sight (darśana) of Arūṇācala destroys all the pāpa (sin) a person has gathered, in this life and in earlier ones. No other qualification is needed. Seeing the hill is itself the cleansing.
Supremacy. Bhagavān Śiva said that Arūṇācala is the foremost among all his kshetras. It is above all other jyotir-liṅga places, above Kāśī in certain respects, and above all other holy places of Bhagavān Śiva.

The jyotir-liṅga of Arūṇācala

Arūṇācala is the place of the first jyotir-liṅga. A jyotir-liṅga is Bhagavān Śiva’s own appearance as pure light. It is not a liṅga set up by human hands or made holy by a pratishṭhā rite. It is Bhagavān Śiva appearing directly in the form of a liṅga. The hill of Arūṇācala is this jyotir-liṅga in its lasting form on earth.

The Śivarātri is established

Bhagavān Śiva’s declaration about Arūṇācala came together with his founding of the Śivarātri festival. After the Liṅgodbhava and the end of the quarrel, Brahmā and Bhagavān Vishṇu worshipped Bhagavān Śiva at this place. That day became the everlasting Śivarātri, on Māgha Kṛshṇa Caturdaśī (the fourteenth day of the dark half of Māgha).

The day when Bhagavān Śiva first appeared as the pillar is recorded as Mārgaśīrsha Pūrṇimā, the full-moon day of Mārgaśīrsha, with the Ārdrā nakshatra. (Vidyeśvara Saṃhitā, Chapter 9)

Kālabhairava’s birth

The birth of Kālabhairava also took place in the setting of the Arūṇācala event. Kālabhairava was made from the brow of Bhagavān Śiva to cut off the fifth head of Brahmā. He later went to Kāśī carrying Brahmā’s skull. This joins Arūṇācala and Kāśī, two supreme kshetras of Bhagavān Śiva.

Sources

Vidyeśvara Saṃhitā, Chapters 7–9 (the origin of the name and the Liṅgodbhava)
Vidyeśvara Saṃhitā, Chapters 8–9 (Bhagavān Śiva’s declaration)
Vidyeśvara Saṃhitā, Chapter 9 (the Śivarātri and the day of the pillar)