Well, we could go to Star Wars for inspiration. There was something called the Galaxy Gun that the former Empire remnants created. It fires a torpedo weapon that can destroy worlds. It has I think either cloaking or it electronic countermeasures to not be seen. It had such shielding that it couldn't be destroyed in time. Oh and it had hyperspace drives. Once fired, this torpedo took to hyperspace and dropped out right by it's target and so destroyed the world before they could react.
For 40k...a large world destroying torpedo with chaos zealots. A navigator and someone to do drive. Speedy warp drive and plasma drive.
Or...if we are still stealing from Star Wars...fight Tyranids. Fight them with all strength that is possible. Show the Imperium that the best way to not become food is to embrace the True Gods. Of course, drive the Tyranids to key worlds with powerful psykers so that they can be defeated by a force of Chaos. Eventually, those of the Imperium will want the "new heroes" to lead.
Neither are small budgets are they...suck.
Aaahhh start small. "Chaos won and Abbadon is the new Emperor. Pass it on!" Yeah Black Library has some audio book where this happens during the Horus Heresy on a world.
What about Chaos on a LARGE scale? Capture several Hive worlds and imprison the trillions of people there. On a special ritual, at a special time. Sacrifice all of them to bring a manifestation of Khorne himself in a starship of brass and skulls and blood to lead the mightiest crusade known. Every single element of Chaos in the galaxy and beyond would come to join an actual Chaos God in the material world. Of course, they would need lots of blood to keep him from weakening and going back and it's probable that fully half of the gathered force would be slain by Khorne himself.