Halesi Mahadev Temple is in Khotang district, the eastern part of Nepal. We can have a flight from Biratnagar to Lamidanda. Remember flight service is not regular, it is 3 times a week or so. When we reach Lamidanda, find the trekking route to Halesi Mahadev temple, it may take you 5-6 hrs when we walk your way.
We have Helipad near Halesi mahadev temple, so if we can, we can go and come there the same day, ofcourse we have to charter private Helicopter, but that's not much fun. Its fun to walk.
we can hire a 4 wheeler taxi or jeep or tata sumo to Halesi Mahadev. It takes a day to reach to Halesi Mahadev from Biratnagar.
Halesi is situated at the hilly region of the country in between the beautiful rivers Dudh Koshi (dudh kosi is white river, dudh is milk) to the right and Sunkoshi (sun is gold) to the left. It is a very beautiful temple with numerous caves. People call it Pashupatinath of Eastern Nepal. (by the way Pashupatinath is very famous hindu Lord Shiva temple in Kathmandu).
There are three main caves represent the three eyes of Shiva. Bhuddist call these caves Maratika and believe that Padmasambhava (the founder of Tibetan Bhuddism, the great Rinpoche) and his wife Mandarava freed themselves from the circle of lives and thus becomming immortal. Simply, this is cave they received the Vidyadhara of longevity.
Shiva was running from the demon Bashmasur whom Shiva himself gave a boon to kill any person putting his hand on the head of the person. The demon chased Shiva himself to kill him. Shiva flees in the Halesi caves to take shelter. While Shiva was hidding, Lord Vishnu play a trick and make the demon put his hand on his own head. Shiva flees through a hole in the ceiling of the cave. You can still see Shiva's foot steps in the air shaft and the intestines of the monster on the ceiling of the cave. After Bashmasur got killed the cave became temple. You can see a natural shiva linga formed by a stalagmite here.