Igor Karkaroff is the headmaster of Durmstrang, a wizarding school. A tall, thin man, he has short white hair and a goatee that does not entirely hide his weak chin. His teeth are rather yellow, and his eyes are cold and shrewd.
Professor Karkaroff disembarks from the Durmstrang ship, and walks up to slope to greet Professor Dumbledore. He shakes hands with Dumbledore, and looks up to the castle with a smile that doesn't quite reach his eyes. Karkaroff beckons Viktor Krum, one of his students, forward after he has greeted Dumbledore, to hurry him into the school as he has a slight cold.
Karkaroff is one of the five judges of the Triwizard Tournament, alongside Madame Maxime, Professor Dumbledore, Ludo Bagman and Barty Crouch. He enters the Great Hall with Professor Dumbledore and Madame Maxime after the students are all seated. After the feast, Karkaroff bustles up to his students at the Slytherin table and tells them to make their way back to the ship. He has a paternal air when he speaks to Viktor Krum but doesn't treat his other pupils in quite the same manner. As Karkaroff leads his students towards the doors he glances to his side at Harry, who is letting him pass. Karkaroff freezes when he sees Harry, staring at him as though he cannot believe his eyes. His eyes move slowly up Harry's face, fixing upon his scar. Karkaroff is horrified when he realises that Professor Moody is in the castle; a terrible look of mingled fury and fear comes over his face when Moody approaches him, and he stares at Moody as though unsure he is really seeing him. He sweeps his students away with him after Moody tells him to move from the doorway, leading them towards the Durmstrang ship.
Karkaroff sits at the staff table on Dumbledore's right for the Hallowe'en feast. He looks tense and expectant as he waits for the Goblet of Fire to makes its decision on the three Triwizard champions. Karkaroff is thrilled when Viktor Krum is chosen as the champion for Durmstrang; he congratulates the boy so loudly that everyone can hear him, even over all the applause.
Professor Karkaroff enters the room off the Great Hall after Professor Dumbledore, accompanied by several other teachers. He wears a steely smile as he questions Dumbledore, and gives a short and nasty laugh as he asks how there have come to be two Hogwarts champions selected. He is angry that Dumbledore's Age Line has not prevented a younger contestant from being selected, annoyed that he could have brought a wider selection of candidates. He appeals to the Ministry officials, Ludo Bagman and Barty Crouch, and insists he should be able to resubmit the names of the rest of his students into the Goblet fo Fire. He wears an ugly look on his face as he tells them they must set up the Goblet of Fire again so that Beauxbatons and Durmstrang are able to have two champions as well. When this proves impossible, Karkaroff says that Durmstrang will not be taking part in the next Tournament, and is of half a mind to leave Hogwarts immediately. Karkaroff is angered by Professor Moody's input into the conversation, and tells him that he will be lodging a complaint with the Ministry of Magic and the International Confederation of Wizards. He dismisses Moody as a paranoid old man, and doesn't value the suggestion that someone entered Harry to harm him, not to advance Hogwarts' chances of winning the Tournament. Karkaroff leaves the room in silence with Krum in tow, livid that there is no option but to accept Harry as a fourth Triwizard champion.
Professor Karkaroff joins the other judges at their table for the Wand Weighing ceremony. Karkaroff twirls his goatee around his finger during the photographs portion of the ceremony, trying to give it an extra curl.
Sirius tells Harry that Karkaroff was a Death Eater. He was caught by Alastor Moody and was sent to Azkaban but was released after doing a deal with the Ministry of Magic. He told them that he had seen the error of his ways and gave them the names of other Death Eaters to secure his release. Karkaroff is not popular with many of the inmates at Azkaban. Sirius says that Karkaroff has been teaching the Dark Arts to every student that passes through Durmstrang.
Professor Moody says that Karkaroff will certainly have told Krum everything he can about the first Triwizard Tournament task. He and Madame Maxime want to beat Dumbledore, and prove that he is only human. Karkaroff awards Harry four points out of ten for his performance against the Hungarian Horntail. His mark is considered biased by Ron, as he awarded Viktor Krum ten points.
Professor Karkaroff leads the Durmstrang students into the Great Hall for the Yule Ball. Karkaroff wears an expression remarkably like Ron's as he watches Krum and Hermione draw nearer to the top table for dinner. During dinner, he discourages Krum from talking about the Durmstrang Institute too much, as he doesn't want Krum to give away its secret location.
After dinner, Karkaroff is overheard in the gardens by Harry and Ron, talking to Professor Snape. Karkaroff sounds anxious as he insists Snape cannot pretend what they are talking about isn't happening. He tells Snape that 'it' has been getting clearer and clearer for months, and that he is getting seriously concerned. He looks discomposed when he spots Harry and Ron standing on the path and he hurries away after Snape.
Karkaroff does not look pleased to see Harry when he arrives at the lake, his expression clearly showing that he was hoping Harry wasn't going to turn up for the second task. Karkaroff is the only judge who doesn’t feel Harry's performance in the lake merits full points, which affects Harry's final score for the task.
Karkaroff knocks on the door of Snape's Potions classroom towards the end of the fourth-year lesson. He looks agitated, and twists his finger around his goatee as he walks up to Snape's desk. He is determined to speak to Snape, and tells the teacher that he wants to talk immediately so he can't slip off, certain that Snape has been avoiding him. Karkaroff hovers behind Snape's desk for the rest of the double period, intent on preventing Snape from slipping away at the end of the class. After all the students appear to have left the classroom, Karkaroff pulls up the left-hand sleeve of his robe and shows Snape something on his inner forearm. He is worried that what is on his arm has never been so clear, and is sure that Snape must have noticed as well. When the two notice Harry in the classroom, Karkaroff turns on his heel and strides out of the dungeon, looking both worried and angry.
Professor Karkaroff is summoned to the edge of the Forbidden Forest after Viktor Krum is attacked there. He is furious when he hears that it was Mr Crouch, a Ministry official, who attacked his student, and believes that there is a plot to put his Triwizard champion out of action. He accuses Dumbledore of luring the Durmstrang delegates to Hogwarts under false pretences, and spits on the ground at Dumbledore's feet. He is hauled against a nearby tree by Hagrid after this insult, his feet dangling in mid-air as Hagrid holds his fist against Karkaroff's throat. When Hagrid releases him, Karkaroff slides down the trunk of the tree and lies slumped at the bottom.
It is revealed in one of Dumbledore's Pensieve memories that Karkaroff was convicted of being a Death Eater after the First Wizarding War. He is brought from Azkaban to give evidence to the Ministry of Magic, having claimed to have important information about his fellow Death Eaters. During the hearing, he is chained to his seat in front of the Council of Magical Law. He offers to assist the Ministry in catching other Death Eaters, having done a deal with Mr Crouch: if he reveals the names of enough unknown Death Eaters, he will be released. Karkaroff grows increasingly desperate during the hearing as it becomes clear that the names he has are already known. He eventually reveals one unknown Death Eater to the Council: Augustus Rookwood. Karkaroff is dismayed when he realises that one name might not be enough to free himself, and offers up Severus Snape as one more Death Eater. He strains against the chains holding him to the chair as he desperately tries to persuade Mr Crouch of Snape's guilt, but is told that Snape has been vouched for by Albus Dumbledore. At the end of the memory, Karkaroff is returned to Azkaban pending a review of his case.
Professor Snape reveals that Karkaroff fled when he felt the Dark Mark on his arm burn, and realised that Voldemort had returned. Karkaroff fears the Dark Lord's vengeance, as he betrayed too many of his fellow Death Eaters to be sure of a welcome back into the fold.