Kerrigor is the name of Prince Rogirek, a powerful Necromancer and Free Magic sorcerer. His name Kerrigor is 'Rogirek' backwards, a nickname given by Touchstone in their youth. During Sabriel, he had plans to destroy the Great Charter stones so that he could summon a massive host of Dead through the permanent gate that the broken stones would have created. He appears to be an immensly powerful dead spirit, far stronger than even Chlorr as a greater dead being.
Early life
Kerrigor used to be called Rogirek. He was a prince of the Kingdom before the Interregnum and, subsequently, Touchstone's reign. He was a childhood friend of his maternal half-brother Touchstone and they played together. This was when Touchstone gave him the name Kerrigor.
Kerrigor
When he was old enough, Rogirek went travelling around the Old Kingdom in order to learn more about Free Magic. During this time he became a Free Magic Sorcerer and possibly killed himself. In Death however he hid his body in Ancelstierre and used Free Magic to make himself appear human.
With this done, he went back to his home and persuaded the queen, his mother, to accompany him to the Reservoir where the Great Charter Stones were kept, with Touchstone as part of the Royal Guard. When they arrived however, Kerrigor had already kidnapped his sisters and made his servants sacrifice them in order to break two stones. He then killed his mother and collected her blood in order to break a third, but Touchstone went berserk in an effort to stop him. He killed the other two guards, who were working for Kerrigor, but failed to stop his childhood friend. Kerrigor was stopped however by the then Abhorsen who appeared, with Mogget in his released form, and sent into death.
Events of Sabriel
Shortly after Sabriel was born, Terciel followed the child's spirit into death (both she and her mother died at childbirth). In Death, Kerrigor managed to take hold of the child Sabriel. He revealed that an inexperienced necromancer had freed him from the gates. Kerrigor held baby Sabriel hostage for Terciel's life. The Abhorsen Terciel rescued Sabriel and managed to seal Kerrigor in death again, Kerrigor claiming that he would return.
Kerrigor returned like he said and lured the Abhorsen Terciel to the reservoir again, aiming to use his blood to break another charter stone. Terciel had to make a diamond of protection and so Kerrigor was only able to trap him in death, actually lodged inside the Fourth Gate.
When Sabriel, Touchstone, and Mogget finally came to the reservoir, Kerrigor made his appearance shortly after Sabriel went into Death, leaving Touchstone and Mogget on guard. Kerrigor's pet Mordicant attacked the diamond of protection that they were hiding inside, but could not get through. Kerrigor advanced on the pair, and they were almost sure they would die.
They were saved, however, once Sabriel freed her father, Terciel, from Death. Once in Life, Terciel took the seventh bell, Astarael, from Sabriel and unbound Yrael to combat Kerrigor. Terciel used Astarael and sent all of the Dead Hands, the Mordicant, Kerrigor, himself and Yrael into Death - as the ringing of Astarael is supposed to do.
Some time later, Sabriel meets the Clayr and she sees Kerrigor in death. He is charging through the gates, smashing them brutally the same way he did before (as noted by a dead spirit) and claimed he would kill her soon.
Kerrigor emerges from Death near to the Wall with a massive host of Dead. He crosses and makes his way to Wyverley College, where he encounters Sabriel, Touchstone and the force of soldiers she had roused from the Crossing Point. Sabriel and the other Charter Mages attempt to cremate Kerrigor's physical body, thus preventing him from returning to Life once he is defeated. Kerrigor and Yrael both smash into the hall where the defenders are before the cremation, and both attack Sabriel. Yrael insists that the Abhorsen is his to kill. After an argument, Yrael attacks Kerrigor, but Kerrigor consumes Yrael. As Kerrigor recovers, Sabriel takes the opportunity to ring Saraneth and Kibeth. Instead of willing him to Death, she binds him into his original body and Kerrigor is forced back into being Rogirek.
When he wakes up, his scream shatters the bells and Sabriel stabs him with Abhorsen's sword, but to no avail. Kerrigor then climbs out of the sarcophagus containing his body and goes to kiss Sabriel. He gets a kiss on her cheek, but Sabriel manages to place Mogget's ring over Kerrigor's head. Because Mogget, as Yrael, was consumed by Kerrigor, his binding is able to affect the both of them. As it begins to bind him, Kerrigor removes the sword from his chest and stabs it into Sabriel, pinning her to the floor. After a failed attempt on Touchstone he is transformed into a sphere in the air, half black and half white; and a silver band around the circumference. Sabriel has no Saraneth to bind the two and so chooses Ranna instead. The sphere splits into two cats, one black and one white, joined like siamese twins. The cats break apart into separate beings, both with red collars, both bound now by the Sleeper, Ranna. The white cat is Mogget and the black cat is Kerrigor.
Since then, Sabriel and Touchstone both bound the slumbering Kerrigor beneath the House, with many layers of protective spells and barriers. All of the binding spells that the two great mages knew were used.