"This Mark has been growing clearer all year. Karkaroff s too. Why do you think Karkaroff fled tonight? We both felt the Mark burn. We both knew he had returned. Karkaroff fears the Dark Lord's vengeance. He betrayed too many of his fellow Death Eaters to be sure of a welcome back into the fold."
-- Severus Snape to Cornelius Fudge (GF36)
Igor Karkaroff was Headmaster of Durmstrang Institute and a former Death Eater. Karkaroff turned in other Death Eaters to the Wizengamot in order to save himself from Azkaban. He ran for his life in June 1995 when the Dark Mark burned into his arm showed that Voldemort had returned (GF35). Found dead “in a shack up north” according to Lupin, presumably murdered by Death Eaters, in the first weeks of the Second Wizarding War (late June or early July 1996) (HBP6).
- Karkaroff named several Death Eaters to the Ministry in order to save himself: Snape, Dolohov, Evan Rosier, Travers, and Mulciber (GF30). But they had all been cleared, killed or arrested. So Igor mentioned Augustus Rookwood, an Unspeakable for the Ministry, and that information led to Karkaroff being released. Sirius Black believed that Karkaroff was responsible for many other arrests, and had heard that he "put a load of other people into Azkaban in his place" (GF19). But that may have just been Azkaban-style grumbling and paranoia by Death Eaters who didn't know who turned them in, and hated those who walked free (which is why Barty Crouch Jr. hated Karkaroff)
- Karkaroff was considered a bad influence at Durmstrang, and encouraged a "culture of fear and intimidation" among the students (Pm). Many parents chose to withdraw their children during his time as Headmaster.