As per the temple pandit and literature around. The Sada Shiva Temple is a very anciant temple, approximately around 5500 years old. Originally this temple was made by the Pandavas during their Agyat Vas. In the Dwapar Yug, Shri Dhaumya Rishi wandering through these mountains, once came across this Dhyunsar Mountain and did Tapasya at this mountain and Lord Shiva gave him Darshan at this place.
As per a verdaan to Shri Dhaumya Rishi, people wishes get fulfilled worshipping at this place. Hence this place also got famous by Dhyunsar Sada Shiva Temple. Later, the Pandavas got a Mandir made at this place during their Agyat Vaas.
Later on as per 1008 Swami Shri Omkara Nand Rishi Ji:
In the Northern India's North Kaashi's auspicious area and across the Banks of River Ganga he did regular Tapasya. In the year 1937, Lord Shiva came in the dreams of Swami ji and asked him to do Bhajan in this temple. There after, he kept searching for a long time for this temple and once in 1947, with the help of Shri Shiv Prasad Sharma Dabral Ji, He was able to find this place and found it exactly as per his dream. There after He stayed in a Cave(Gufa) close to this temple and did his regular tapasya.
In 1948, with the help of people He got the temple made and there after at some intervals more developments of the temple were done to what you are seeing today.