Does your alchemy/herbalism system include, or have the capacity to include poison craft? I think that would be the most interactive way to do it. Or , you could have Carrier Crawler venom and Umbral Gray to have an on use to combine with one another to make such a poison.
Alternatively, you could cut out the middle man and just make Umbral Gray a poison in and of itself, and reference the combination of the venom in its description and add it to the loot tables. Probably the easiest of the three options. Con based would probably be your best and fariest bet for that. You don't want something to just go around killing people or offering On Hit:Death if you're coating items with it.
Parts of Book of Souls are really good for spice and filler, but, like all Net Books/Adventures, I'd take them with a grain of salt and not try and base anything too heavily off of them, you risk running into aforementioned ridiculousness and contradictions.