Agasteshwar Mahadev Temple is in the premises of Santoshi Mata temple which is to the back of Harsiddhi temple. As per the description in the mythological writings when devils got victory over god, they started roaming on earth because of sadness. While wandering in the forest they saw great Agasteshwar saint who was shinning like sun. By seeing the condition of god, saint was angry. Because of the outcome of which volcano was erupted and demons started falling from heaven. After being afraid saints and others moved to hell. With this saint Agastya was sad. He went to Brahma ji and said that I have done Brahman killing.

Thus tell me some solution with which my reformation can be done. Brahma said that to the north of Mahakal forest near to the Vat Yakshini one nobel ling is there. By doing prayer of it you will be free from evils. On the narration from Brahma ji, saint Agastya did meditation and Lord Mahakal was pleased from him. Lord gave him blessing that the ling of the god of which you have done prayer will be famous with your name in all the three worlds. The man who will visit Agasteshwar with the feeling of devotion will get all his wishes fulfilled by getting free from all evils. From that time prayers of Agasteshwar Mahadev is done by the devotees in Shravan month every year.

Source:- http://www.apnaujjain.com/137/84mahadev.html