In Blood Omen 2, Kain entered the Eternal Prison. He needed to find the Builder, a being who could tell him how to destroy the Device.
As he travelled through the corridors of the prison, Kain saw a terrified human. The male wretch was fleeing from a mutilated, mad vampire demanding meat. This vampire had presumably been injured by the Prison Guardians - he had a furnace attached to his back, his brain was exposed, and much of the flesh of his torso had been stripped away, revealing the organs beneath.
The mad vampire was stopped by two Prison Guardians. They reminded him that he was "not allowed down there" in that part of the Prison, but he attacked them, thus claiming his entitlement of fourteen hundred ounces or twenty stone of meat per day. The Prison Guardians clearly had difficulty controlling him, and the broken chain hanging from his wrist suggested he had broken free from them; they had not intended to let him run amuck. As the human was surplus to his meat allowance, the vampire threw him into a glass barrier, killing him. The glass broke, allowing the vampire to reach Kain.
Kain could not defeat the deranged vampire with brute strength, so instead Kain tricked him into falling into a water tank. This did not destroy the vampire, and later Kain saw him again, ripping other prisoners to shreds as they lay bound on tables. When Kain saw the vampire attempting to chase him across a bridge, he used his 'Telekinesis' Dark Gift to split it, and the vampire was submerged in water for a second time.
After Kain had met the Builder, he found he had still not shaken off his pursuer. Again, Kain removed a bridge, dropping the vampire in water. A little later, Kain had to negotiate a maze within the prison. Through the bars, he could see the mad vampire approaching whenever he was caught in a trap. Somehow the creature still lived. Eventually Kain was drawn into a proper confrontation with him.
The vampire stood on a rock in the middle of a water pool. Kain had to hide behind statues to avoid the vampire's powerful Dark Gift: 'Immolate'. This gift would have incinerated Kain, but he managed to repeatedly knock the vampire into the pool using 'Telekinesis'. The vampire fled into another area of the prison, and Kain followed to continue the fight. The battle ended when Kain caused a huge statue of Moebius to fall on top of the vampire. He did not appear to be truly crushed, but he had been affected by the impact as the statue shattered around him.
As Kain stood over him, the vampire spoke. He was no longer raving or shouting, his voice had become calm and rational. With the return of sanity, the vampire knew Kain.
He had once been Kain's champion, the finest warrior in his army. Kain now recognized Magnus, but his memories were tainted with suspicion. He believed Magnus had betrayed him shortly before the final battle with the Sarafan Lord, leaving their camp to ally with his enemy. Magnus told him the truth. Proud and overconfident, he had sought only to serve his master. He had gone alone to the Sarafan camp, hoping to kill the Sarafan Lord and win the war for Kain. The Lord defeated Magnus, robbed him of his mind, then sent him to the Eternal Prison as a punishment.
Kain understood the depth of Magnus' loyalty. He granted him the only reward he could - death and a release from his torment. By 'absorbing his veins', Kain acquired the Dark Gift 'Immolate' as Magnus died.
Some fans have noted that fourteen hundred ounces does not equal twenty stone in weight. In fact, forty hundred ounces would be closer. As Magnus really does seem to say fourteen, rather than forty, we should probably assume he was confused. He was mad after all.