There used to live a bheel named Aahuk on the Arbudachal mountain. His wife was Aahuka and both of them were supreme devotee of lord Shiva. Once, lord Shiva wanting to test their devotion, appeared before them disguised a hermit Yatinath. Aahuk honoured his guest and treated him very well. Lord Shiva then requested him to give shelter for the whole night. Aahuk expressed his inability as he had a very small hut, in which only two people could be accommodate at a time.
But his wife intervened and requested Aahuk to sleep outside the hut with his arms, as it would be inappropriate on their part to miss this chance of proving their hospitality. The hermit, Yatinath who in reality was lord Shiva and Aahuk's wife slept inside the hut, while Aahuk himself slept outside. Unfortunately Aahuk was killed by a wild animal while he was asleep. In the morning when lord Shiva found that Aahuk had died, his heart was filled with grief.
But Aahuka consoled Yatinath and decided to give up her life by jumping into the burning pyre. Right then lord Shiva appeared in his real form and blessed her by saying “In his next birth your husband would take birth in a royal family. He would become Nala and you would be born as Damayanti. I would myself appear in the form of a swan and help both of you to unite.” After saying this, lord Shiva established himself as immovable Shivalinga, which later on became famous as Achaleshwar linga.