The story behind THIRUMANDIRAM.
Thirumoolar was a yogi by the name of Sundarar from the Mt. Kailas area of the Himalayas.
He decided to undertake a journey to see a friend and fellow yogi. This friend resided in the Potheya Mountains of south India. He was known as Yogi Agastyar.
Nearing the end of his journey, near the village called Sathanur, he saw a sight that deeply moved him. He noticed a herd of cows lowing miserably around the dead body of their cowherd, Mulan.
This yogi who knew well..."Anbe Sivam" (God is Love), felt he should do something to alleviate the suffering of these poor creatures. Through the use of his yogic powers, he put his body in a hollow log for safe keeping and then left it to enter the body of the dead Mulan.
The cows were overjoyed to see their master returning to them. He then led them back to the village. After leaving them safely at home, he returned to the old log where he had left his body. He was shocked to find that his body had disappeared.
This yogi remained in the cowherd's body and became famous for the verses he uttered when he occasionally would come out of Samadhi (contemplative absorption). In this fashion, 3000 verses were recorded, and these are now known as THIRUMANDIRAM.
Source...
http://www.thirumandiram.net/about-tirumular.html
திருமூலர் அருளிய திருமந்திரம்
Thirumular Thirumandiram
http://www.shaivam.org/siddhanta/thm.html