Koteshwar Mahadev Temple is a Hindu cave temple dedicated to Lord Shiva, and is located about three km inside the ‘heart’ of Rudraprayag district of Uttarakhand on the holy bank of river Alaknanda.

The legend has it that when Lord Shiva was hiding himself from Pandavas (As the Pandavas were seeking repentance from Lord Shiva after killing Kauravas) and was heading towards Kedarnath, he chose to stay and meditate for sometime in Koteshwar. Another legend attached to the temple is that Lord Shiva hid himself here from Bhasmasur Demon. It is said Lord Shiva meditated here and found favour by Lord Vishnu, who in turn killed Bhasmasur in his Mohini Avatar. This temple thus is an important religious destination for Hindus.

There are many naturally formed Shivlingas in the cave on which water drops on these Lingas continuously. During the months of August and September, thousands of devotees come here to worship Lord Shiva.

Source:

https://www.tourmyindia.com/blog/must-see-locally-popular-temples-in-uttarakhand/