Hi all,
This situation came up recently in my group when I gave one of the players the option of replacing his trice-blessed chainsword with the same quality chainsword with a minor daemon in it, to represent his character's embrace of radicalism and because as a fallen priest, he would probably suffer from some degree of self-hatred and not want such a holy weapon to be defiled. Instead he handed me his character sheet to inspect, still with the holy chainsword and now with a daemonically possessed force staff.
Needless to say I was not impressed. As the GM I ruled that the two traits are effectively incompatible. While the rules do not say that you can't put a daemon into a force weapon, this is a bad idea for two reasons:
1) Mechanically, you'd be getting the bonuses of your character's WP and that of the daemon to both damage and pen. You'd also have the force weapon's ability and those of the daemon weapon stacking. This is obviously not a good thing in terms of game balance.
2) A Force weapon only works because it is made of the finest psychically-conducting materials known. If it allows the psyker to channel psychic energy into it, it follows that psychic energy (daemonic essence) could easily flow through the blade and into the psyker's mind, leaving him utterly open to possession.
What are your thoughts on this?