The Builder

In Blood Omen 2, when Kain first visited the Device, he encountered the Beast (Janos Audron). The Beast assisted Kain, knowing that if Kain succeeded in destroying the Device, it would be freed.

The Beast told Kain to find the Builder of the Device. Kain found it hard to believe that an individual so ancient could still be alive, and he said so. The Beast replied that the Builder could be found in the Eternal Prison. The time distortion there made it possible, as hours and years were frozen, causing the prisoners to pay for their crimes eternally.

Kain entered the Eternal Prison, ignoring the attempts of the Prison Guardians to turn him away. Finally he lowered a cell which held a Hylden prisoner. The Hylden crawled out and Kain kicked him. He questioned the prisoner about the whereabouts of the Builder.

The prisoner was the Builder. He bargained with Kain - he was happy to tell him about the Device in return for an end to his suffering. The Builder had been sent to the Eternal Prison while the rest of his race was banished to the demon dimension, and his fate was conceivably worse than theirs. Kain agreed, and the Builder told him about the purpose of the Device and the abilities of the Mass. Kain realized that the Sarafan Lord intended to use the Glyphs as a conduit for the lethal mental energy of the Mass.

When the Builder learned that the Device might finally be used, he showed concern. He no longer wanted his creation to unleash its destructive potential. Asking Kain to "kill that which [he] created out of arrogance and pride", he offered his blood to Kain, so Kain could use it to poison the Mass. In taking the blood, Kain killed the Builder to fulfil his obligation to him.

FURTHER OBSERVATIONS

There doesn't appear to be a reason why Kain specifically needed the Builder's blood to poison the Mass. Kain claimed he could feel the Builder's blood "course coldly" through him, but at the rate Kain's recently-awakened body consumed blood, there would have been none left in his veins when he reached the creature. It seems more realistic that any Hylden's blood would have done the job. Kain was on a Hylden-rich diet throughout the Device, and the poison certainly didn't appear diluted - the Mass died almost instantly when exposed to it. It's likely the trip to the Eternal Prison was only necessary for Kain to learn the Mass' weakness in the first place.

Alternatively, if we take Kain's comment as fact, and the Builder's blood was special, it may have been a consequence of his imprisonment. Perhaps an element of 'impurity' was introduced into Hylden blood in the demon dimension, leaving the Builder as a final 'pure' example of their race (besides the Seer?). The comparison with the Seer inevitably brings up one last observation about the Builder: somehow he had become as emaciated as his banished kin, while the Seer had not. There is no obvious explanation for this - the emaciated appearance of the Hylden was supposedly a consequence of their banishment; the demon dimension 'rendered them hideous'. If she kept her looks, having apparently avoided their banishment, surely he should have too.