Adorjan Megidézó was a Magus of House Bonisagus, one of the founders of the Covenant of Riversedge some 35 years ago. Hailing from the Transylvania Tribunal, he was Marched in the not-too-distant past for Diabolism and for murdering an ungifted Redcap.
Katerina, the apprentice who would become Cygna, was one of the things that, while he was not ever censured at Tribunal, seemed to have skirted the Code. He allegedly abused the portion of the Oath that concedes the right to take an apprentice if he finds her valuable to him in his studies." While traditionally, that clause gave the Bonisagi the right to take the apprentice of another magus for their own, he argued that he had the right to "borrow" her for a season at a time, whenever he wanted, with the overt thread that if he were not accommodated, he would simply take her outright. It was during those seasons under Adorjan's tutelage that she grew Wrathful and acquired her vulnerability to the Diabolic (either that, or Adorjan exploited an already-existing weakness and made it greater). It was also during her seasons with him that she saw enough of what he was doing that, once she had learned enough, put together the pieces and gave the information to the Quaesitores that damned him. (basically, she had seen x, y, and z in the lab, during his work and so on, but didn't know enough to come to the conclusion that x+y+z=holy darnation until some time later). At his trial, when he was condemned, he swore that Cygna would never be rid of him. It was suspected that he had not worked alone, but they never found any evidence of any fellow diabolists.
It was after he had been Marched that she started seeing, and dealing with, what she believes to be his shade.
He also shows up in 2.1 Prelude - Cygna (Part 4).