When a vampire gives a witcher a contract to kill another vampire, you can be certain neither of the two is entirely normal.
The contract seemed simple enough, though Geralt knew his target was a vampire. Yet one must bear in mind vampires are a very diverse group and their various species differ from one another greatly.
The witcher established the individual he sought had long, hooked claws with which it could tear apart a body with ease, leaving behind venom which hindered the clotting of blood. What's more, it could use these claws to scale even the sheer outer wall of a building, giving others the impression it possessed mind-boggling powers. Geralt also realized his opponent was an experienced predator able to set ambushes for its prey and attack it unexpectedly.
The monster he was tracking turned out to be a powerful type of garkain, a pack leader or alpha.
As the witcher expected, in addition to its ability to emit waves of mental energy, this alpha garkain also had powerful venom at its disposal.
The monster killed for pleasure, not just to satisfy its hunger. It must be admitted that Geralt also gained a measure of satisfaction from eradicating this beast, for he could still remember the massacre in the orphanage for which this garkain was responsible.