In Blood Omen, Nupraptor was the Guardian of the Pillar of the Mind. He was a master of telepathy and telekinesis, and pilgrims would travel across Nosgoth to visit him. He was 'hypersensitive to peoples' emotions and thoughts'. He was also 'prone to rash action and sometimes unpredictable and illogical decision making', but passionately loyal to people he cared about.¹
Before Kain's birth, Nupraptor had been the lover of Ariel, the Balance Guardian. Apparently he adored her. When Ariel was murdered, Nupraptor plunged into a turmoil of suspicion and grief, suspecting treachery within the Circle itself. He went mad, and unleashed a telepathic shockwave on his fellow Guardians, infecting them all with his insanity. The Guardians' symbiotic link, which normally bound them together into a strong whole, served to bring down the entire Circle. The Pillars of Nosgoth began to collapse, reflecting their Guardians' derangement.
Madness affected the Guardians in different ways. Nupraptor withdrew into his retreat. It was an imposing skull-shaped building, located on a cliff top above Vasserbünde. In his self-imposed solitude, he mutilated himself, "sewing his eyes and lips shut to deny the outside world." The Mentalist turned on his own servants, the pilgrims entered but did not leave the retreat, and he was regarded as a lunatic by Vasserbünde's citizens.
When Kain - as a fledgling vampire - first visited the Pillars, Ariel told him about Nupraptor. Kain then sought the Mentalist, intending to kill him. He entered Nupraptor's Retreat, and eventually he found Nupraptor with Malek. Luckily, Malek was quickly dismissed - the Mentalist did not want his protection. Kain judged Nupraptor as a "broken, pathetic little man". His head was bulbous and deformed; his green upper-cranium pulsated. He never moved from his chair, "yet crippled as he was, he would not yield without battle". Part of this battle included teleportation, and the Mentalist telekinetically controlling skulls to circle and shield him, then launching them across the room to hit Kain. Kain fought Nupraptor, decapitated him, and claimed the Guardian's severed head. He returned the grisly trophy to the Pillar of the Mind, thus restoring it (Nupraptor's head was his Pillar token). However, Nupraptor's influence on Kain's destiny did not end there.
After slaying the remaining Guardians, Kain discovered that he was Ariel's replacement. He was the representative of the Pillar of Balance, born as she died. Like the rest of the Circle, he had been corrupted by Nupraptor's magic. Kain realized that he would carry the effect of the Mentalist's madness for the rest of his life...
Nupraptor was mentioned twice in Soul Reaver 2, firstly by Kain at the Pillars. Having travelled back in time, Kain described the murder of Ariel and the resulting derangement of the Circle as it was happening. Raziel witnessed the corruption of the Pillars as the dark waves of Nupraptor's psychic attack swept across the clearing. Old Kain tensed and looked a bit unsteady as he weathered the blast, although it didn't seem to affect him beyond that - all the damage that could be done to him had already been inflicted on his newborn self. The effect of Nupraptor's magic was not cumulative.
The second mention of Nupraptor came when Raziel met Ariel at the toppled Pillars, over a century later. When she asked him to be merciful, and stop 'hounding' her (he had followed her into the Spectral Realm), he cruelly asked if she meant the brand of mercy she showed her "beloved Nupraptor" when she "made him Kain's first kill".
It is worth noting that the Players section of the Silicon Knights Blood Omen FAQ claims Nupraptor had singled out Mortanius as Ariel's murderer. This was before he wreaked his indiscriminate retribution on all of the Guardians. Also, it was intentional retribution as suggested by Blood Omen, and not just an uncontrolled overflow of Nupraptor's madness, as could have been inferred from Kain's Soul Reaver 2 description. Nupraptor might have attacked the whole Circle like this, knowing that he could never strike Mortanius directly; the Necromancer was too powerful for that. The same source says: 'All across Nosgoth, the minds of the sensitive and intellectual are blasted by relentless waves of hatred', hinting that Nupraptor's attack might have affected quite a few people outside of the Circle too.