I’m currently running a Dark heresy game with a band of radical acolytes. Their Inquisitor, unbeknownst to the acolytes, is a Phaenonite working on constructing a part-daemon, part-AI device capable of calculating the true names of Daemons upon entering real-space so that they can be bound and banished (or potentially forced to serve as Daemonhosts or somehow aid other Imperial interests). To contact his acolytes, he uses his Interrogator to pose as their inquisitor and deliver messages.

However, unbeknownst to their Inquisitor, their Interrogator is acting as a double agent for Chaos, and is looking for a way to smuggle the device to Chaos Forces for profit and power. Additionally, the Phaenonite Inquisitor’s rival, an Ocularian, has discovered their Inquisitor’s plan and is trying to expose him.

I have two questions for yall:

Question 1: What kind of ratio of plot to side adventures should there be? I feel for the sake of variety, it may be a good idea to interject missions unrelated to it to give them some breathing room.

Question 2: Which Chaos Faction should be the most present to the Acolytes? Because it helps to have more frontline antagonists, I was wondering which Chaos Factions would have an interest in acquiring this device from their Interrogator. So far I’ve thought of the following ideas:

Traitor Guard – perhaps a section of Imperial Guardsmen have turned against the Imperium and are desperately trying to sequester the device as a way of reinforcing their diminished numbers?

Chaos Space Marines – any number of Chaos Legions could potentially want this device, but I was thinking that the Word Bearers and Iron Warriors would potentially have the most to gain from it – Word Bearers due to their reverence of daemons and reliance on them as auxiliaries, and the Iron Warriors for the Silica Animus component plus their fetish for daemon engines.

Rival Daemons – Perhaps a Daemon Prince or Herald wants the device so that it can use it against one of its Daemonic Rivals? This could be interesting as it would mean the Daemon would actually be aiding the acolytes, trying to help them assemble the device without risking it being used on them.

Which options do you think work the best – if none of these make much sense, what do you think could be useful?

Don't forget the Hereteks who would be super eager to get a daemonic AI. These could be Dark Mechanicus or just radical renegade AdMech... or why not both?

As for the ratio thing, I've found that it's handy to do one or two minor things between each step of a major thing, unless the players really want to push the main plot along (sometimes if you do a good job of creating a sense of urgency, the players will pick up on that and want to rush the next main quest stage). It works best if you can occasionally seed something relevant to the main quest into the side adventures, or at least making it seem like you did, and always be responsive to players who come up with "obvious" links between major and minor quests - let the players do the work for you. Some of their theories you can reject (but be cagey about it), others you can adapt to your own ends (they were right in principle but got the particulars wrong), and the rest you can just lift wholesale (they were right all along!).