The knife can probably kill anything, immortal or not, and was used to kill Jesus. The grail is a purifier, the Crown of thorns and placard wards, and the shroud resurrects. Together they may have cured one third of the White God of Nemesis.
Look what had to be done for Leas cure a much less powerful being, and Mab killed Maeve rather than attempt a cure.
Why would Nic want the knife? It could kill a Fallen (not the host) and Nic knows he has Nemesis infected Denarians. Perhaps he is even contemplating its use against a fallen Archangel if needs be.
It has been hinted that these items may have older provenance to the era of the Greek boos or earlier which is why Hades has custody of them. The placard could be the earliest book of Magic, the book of Thoth which contained two spells, one to perceive the speech of animals, the other to allow the reader to perceive the gods.
If it is that Then given the popular theory concerning Mister being the White God, Harry has the perfect tool to determine this, and to find out what Mouse really is.