Umapati Shivacharya, one of the three thousand Dikshithars of Thillai, was one of the Saiva Siddhanta Santhana Gurus. He was very proficient in both Tamil and Sanskrit. The Chola king, recognizing his knowledge, had gifted him with a pearl palanquin and a nagara, etc. One day, while he was returning after the darshan of Lord Nataraja, Maraignanasambandar, a staunch advocate of Saiva Siddhanta, was teaching his disciples. Maraignanasambandar's disciples reported that Umapati Sivachariyar was passing by on the street. Immediately, Maraignanasambandar said, "patta kattaiyil pagal kurudu pokirathu!" (on the dead wood the day blind is going!) Hearing this, Umapati Sivachariyar understood the profound meaning of those words and immediately fell at Maraignanasambandar's feet and requested him to accept him as his disciple. Maraignanasambandar also accepted Umapati Shiva, who was highly educated and mature, as his disciple. 

One day, while walking along the weaver's street with his wise teacher Maraignanasambandar, Maraignanasambandar drunk of the porridge used for starching the cloth. The porridge flowed down his elbow. Umapati Sivacharya drank the spilled porridge, considering it as a Prasadam. The Thillai Dikshitas excluded him from their group, saying that he had broken the discipline by eating uchishtam (food with someones saliva). From that time on, Umapati Sivachariyar stayed at a place called Kortravangudi outside Thillai. He studied Saiva philosophy from his guru, Maraignana Sambandar.

Once, although he had the right to hoist the flag during a temple festival, another Dikshitar was given the right to do so, but the flag never went up. Later, he was summoned by some other Dikshitars who realized the greatness of Umapati Sivacharya's devotion. Umapati Sivachariyar sang the verses of Kodikavi with each song the flag rose up towards the top, and in the fifth song, it rose completely to the top. Thillai Vazhanthanar and others were amazed by his greatness.

At that time, there was a Shiva devotee named Petran Sampan. He was engaged in the daily work of providing firewood to the Thillai temple. He was a spiritually matured person who worked without attachment or any regard for the fruits of his labor, contemplating on the supreme Shiva. Lord Shiva came up in his dream and gave a song, “Adiyar Keliyan Chitrambalavan” asking Umapati Shiva to give Deekshai to Petran Sampan and moving him up to liberation. He gave him a letter and ordered him to give it to Umapati. As he woke up as he had seen in his dream, the letter was in the hand of Petran Sampan. Petran Sampan also started providing firewood for Umapati's Mutt. One day, Petran Sampan saw Umapati Sivacharya and humbly offered him the Thirumuga palm leaf letter, which Lord Chitrambalavan had given him, and stood there in homage. Umapati, who read the divine message sent by his worshipped Deity, Adalarasan, initiated Petran Sampan, gave  “Sadyo Nirvana Diksha”. Sampan received the Moksha instantly along with his body.

Unable to realize that her husband had received the liberation, Sampan's wife, complained to the king that Umapati had burned her husband with some mantra. The king also came to the Umapati monastery and inquired. Umapati, a devotee of Lord Shiva, explained the truth. Even when king realized it, he said that Umapati Sivam should perform the miracle again so that his Sampan's wife and others could see it. Umapati Sivacharya agreed and said that since there was no one in that place who was mature enough to attain the liberation, he would use the Muli plant that grew virtuously at the place where Thirumanjana water fell during daily worship to Lord Shiva to bestow liberation. He initiated the thorn plant. The thorn plant also became radiant with light and got liberation. King and others were amazed by this miracle.

He composed eight books on Saiva Siddhanta core texts, which are revered as the Siddhanta Ashtaka. He has also written the history of the temple of Chidambaram under the name Koilpuranam. He has also written the history of Sekkizhar under the name Sekkizhar Puranam. He was well-versed in the Sanskrit ​​as well as Tamil and also wrote a commentary on the Paushkara Agama. Umapati Sivacharya lived in the late 13th and early 14th centuries.

Siddhanta Ashtaka are

1. Sivaprakasham
2. Thiruvarutpayan
3. Vina Venpa
4. Porri Paqrodai
5. Kodikavi
6. Nenju Vidu Thuthu
7. Unmai Neri Vilakkam
8. Sankarpa Nirakaranam

Apart from these he has written Tamil books (These indicate the high reverence this devotee had on Thirumurai)

• Koyil Puranam
• Thirumurai Kanda Puranam
• Thiruthondar Purana Varalaru / Sekkizhar Puranam
• Thiruthondar Purana Saram
• Thirupati Kovai
• Thirupadika Kovai
• Thevara Arunmurai Thirattu
• Gyana Achara Sastra Panchakam

See Also: 
1. meygaNdAr 
2. chEramAn perumAL n^AyanAr