I spoke to Victor Grayson and ordered him to hand over the skull, he informed me it was only useful once and in which he had used it already at Fort Mur, I conducted a heavy search and found no skull and I do not think he hid it as such a big item would be easy to find within his bag.

The story is likely true. I've seen several enchanted skulls. Human bones are not designed to focus that kind of magical power, frequently they are destroyed when its unleashed.

Agent Trepidity Hectra