Pros and Cons of each? Particularly, wreaking a whole sector and turning the Inquisition on the Novamarines?
I don't have anything particularly hardline here, but of course I would work with the concepts given. Nevertheless...
It is well known amongst the warband that, although, of course, he has many impressive achievements to his name, our Demon Prince ultimately achieved his ascension by...
[] The conversion of most a loyalist chapter to the Dark Gods. This is, naturally, where most of the members of our Warband come from.
Most of your marines are better equipped and have better teamwork, as they come from a background of trusting each other and had modern technology. On the other hand, they aren't veterans of 10 millennia of war, and the Emperor is probably a little unhappy with you. On the other other hand,
skill in corrupting loyalists, particularly Marines. I can't articulate more of my thoughts on this. at the moment.
[] The mass offering of a large number of Eldar Soulstones. Slaanesh strongly supports our warband, although our Prince is nevertheless of Chaos Undivided, as is true of all Dark Apostles.
Significant support from Slaanesh, generally. Compared to some other options, the Emperor is less pre-disposed to hate you, since you haven't slaughtered imperials particularly. The Eldar would
hate you though, more than usual for being Chaos. Skill at fighting the Eldar, probably other things I'm not thinking of right now.
[] The destruction of a bothersome Tyranid Splinter Fleet. He even corrupted a few of the lesser creatures, which have remained a useful tool of our warband.
Balanced support from the Chaos Gods, and the Emperor might be
ambivalent about you instead of naturally erring on the side of hate, plus, obviously, having some Tyranid creatures with theoretical potential to experiment and tinker your way to the big stuff. Would probably be (relatively) easier to draw over Ultramarines with this, what with them getting fucked by a hive fleet. Prince knows
how to corrupt Tyranids, to some degree. Probably other details.
Write-ins
[] By his hand a full sector fell to chaos, civil war and discord within a century. When imperial contact was re established they found petty emperors and cultists fighting amongst themselves.
Significant support from Khorne (blood for the blood god), moderate support from Tzeench (likes plots) general support from the other gods, skill in stealth and sowing disorder... On the other hand, the Imperium would treat you as a high priority whenever they knew you were there, and the Emperor would be
very unhappy with the human blood shed. Or so your warband believes. Probably other details
[] Through events he set into motion, exterminatus was declared upon the world of Honourum, crippling the Novamarines as a standing force and sowing distrust between the Ultramarines and the Inquisition.
Perhaps not as dramatic as taking an entire sector, but I can see some appeal to the Four. The Emperor, perhaps not so much...
High support from Khorne, Tzeench, and Nurgle, general skill at manipulation, the Alpha Legion probably likes you, Emperor
displeased with the destruction of a planet... Probably other details.