This is Horus Heresy level material.
Grammaticus is a perpetual, an unkillable depending on the age...although that was well before Dark Hersey was ever set in either version of the games.
Both games taking place in the 41st Millenium and Grammaticus was last heard of in the 31st millenium. He was supposed to end up setting events in motion to get Vulkan killed, but...well that obviously didn't happen.
Never heard of Dorgun before though in...well anything.
Mon-Keigh however seals the deal of universe.
Still unknown on which game.
Possible game setup. The group are working for the Cabal, a shadowy Xeno organization that utilizes Farseeing techniques to divine the future and who actively work to make the futures they want a possibility. Basic level Illuminati stuff. They obviously work heavily with the Eldar for their psychic aptitude at Farseeing techniques.
If Grammaticus is a matter to worry about then it's got to be set during the HH, because that's when the Cabal conscripted John (his first name) into service.
The one they cannot see might be referring to a Culexis Assassin, an Untouchable/Blank. They can't be divined upon because they have no warp presence.
No idea which sword they're talking about though.
If they're going 40k it'll likely be easiest for them to do Dark Heresy 2nd Edition. And this of course obviously being set in a different place/time than Rollplay: Dark Heresy.