Given that Nightslayer's true Serioli name actually translates to "Loci for different levels of energy from various planes of existence", it seems quite likely that this weapon has powerful Necromantic powers.
Nightslayer may possibly be the most powerful of the Great Weapons so far revealed. At the least, its psychic presence is described by Vlad as being even more predatory, vicious, and powerful than that of Blackwand, the high-water mark of Great Weapons to date.
- In Yendi (book), Chapter 10, Vlad talks about Iceflame and says, "The only other known artifacts with power to match it were the sword Godslayer and the Imperial Orb." So, neither Nightslayer or Blackwand are in the running. I don't think psychic presence is directly related to potential power, at least not with Great Weapons.
- It should also be considered that "Nightslayer" may mean "Slayer in the night", a predator, or "Slayer of the night", a star.