Here's my proposal:
First, meet with a handful of our most trusted, most loyal commanders and order them to start using encrypted vox channels to discuss plans to desert our crusade and flee the system. I trust the Machine God's SIGINT division is good enough to decrypt their mutinous mutterings, and the resulting disinformation will make our erratic behavior in the rest of this plan seem more believable. It's the sort of behavior you'd expect from the Highly Dependable Marines, anyway.
Second, I just remembered: our flagship is a refit of Abaddon's old battle barge.
This is Abaddon's old battle barge. I note that it is called the Planet-Killer and, in a rare lack of duplicity, has a planet-destroying gun. Terra apparently cannot be destroyed without dire consequences for all mankind, but the rest of the planets and moons in the system are fair game.
So, if the Ordo Malleus isn't airborne yet, deploy a large number of ships piloted by skeleton crews to orbit over Titan. With any luck, the Grey Knights will prepare to repulse the anticipated assault. When the Tribunal's in range, instruct all "invading" ships to scatter, and obliterate Titan with the Armageddon Gun. Alternatively, if the Grey Knights have already taken to the skies, or if Titan's aerial defenses render it impractical, try the same trick on Mars. (We could even attempt to marshal Magnus's bros and our own now-godlike sorcerors to create illusionary ships, or cloak the Tribunal; this will augment the apparent invasion force and bait VD to deploy more troops to planetary defense. I forget whether pariahs can disbelieve illusions, though; if they can, discard that idea.)
Depending on how the gun works, we could take out Titan AND Mars. In this wretched system and across the galaxy, let the False Emperor's worshippers look upon his most loyal servants and see that the fruit of their faith is death.
Other than that, though, I suggest caution, and that we avoid direct engagement until Iblis arrives. Our planet-nuking tricks are flashy and all, but we still can't win a straight fight on land or in the air, and mankind can't afford for us to get shot down before the battle on Terra begins.
I do recommend that once Iblis hits Terra, we waste no time waiting for a clear victor to emerge. My guess is that when the hive fleet is readying for invasion, the Emperor will recall nearby ships to Terra's defense. We should move in then and pick off as many as we can when they're en route; the ones that make it to Terra will be caught between us and the Iblis. If we make it that far and the Swarmlord's plan with regard to VD does indeed turn out to be DEVOUR SOUL!!!, I suggest we harass the Terran defenses; this will either force the Emperor to divert forces from the Swarmbro, or he will ignore us and allow us to establish a beachhead on Terran soil.
Keep the scroll on Jeremiah's person at all times. Use it if things are dire and conventional means of destroying VD seem implausible. If he tries to DEVOUR our SOUL, he'll choke on it.
