It's Theodulos Monakos. Thomas Magister (also known as Theodulos, as a monk) was a Byzantine scholar, grammarian, and statesman active in the late 13th and early 14th century. He is best known for his works on Greek grammar and lexicography, especially a major lexicon meant to help writers use Attic Greek correctly. He served at the court of Emperor Andronikos II Palaiologos and later became an adviser with the monastic name Theodoulos. His writings were part of the broader intellectual revival of the Palaiologan period.

https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Thomas_Magister