Basically he was the creator of Thorn, Dredgen Yor was a fake name, there are a bunchbof theories of who he really was, some say Pujari, some say Jaren Ward. Basically, he was once one of the best Guardians, until he becamed depressed and turned to darkness. He used thorn to kill other Guardians such as Thalor and Pahanin. The only difference between his thorn and our thorn is his burned people forever.