Hello fellow heretics!

Before i start i want precise that i play Black Crusade in 821M41

I run a sandbox campaign and last session, my players decide to travel from Screaming Vortex until Vraks ( Scarus Sector ), so according to w40k wikia, it's Azrael the Supreme Grand Master who lead half of the Dark Angels on Vraks. But Azrael become Chapter Master in 939M41, so who really lead the Dark Angels during this war?

Also i have a few questions about the strength of the chaos warband of Arkhos, how many marines are they? What troops compose this warband?

The same questions about the warband of Zhufor ( The Skulltakers ), it's said that it's one of the largest World Eaters Warband, but what does it mean? A Thousands? A Hundreds?

The loyalist fleet is always in orbit? If yes how many/powerfull is the fleet?

And if you have a few advices to help to run such campaign, i'm open.

Well, as per the codex, Azrael's predecessor was called Naberius, so if it was not Az', then maybe him?

Codices and Forgeworld books are written by different studios, and so - much like with novels or this RPG - may not be consistent with one another, as these people have different ideas and are not required to reference one another. " There is no canon " means that you are free to choose which of the conflicting sources you want to go with here, just like you already make stuff up simply by playing a game in this setting (and like you will have to do for most of your other questions). So I'd say first you have to decide whether you want to modify codex fluff to conform to the Forgeworld book, or FW fluff to conform to the Dark Angels Codex.

Make sure to re-check the actual sources to be on the safe side, however. Wikis are, in the end, fan-edited sources, which makes them even less reliable and consistent than the official books.

As for Chaos Warbands, I'd say "hundreds" is plenty large. After (approximately, because Warp) ten millennia of constant warfare and countless internal conflicts, organisations such as the World Eaters will be a far cry from the Legions of old, and instead more resemble small but incredibly terrifying independent mercenary companies, in all likelihood with their own posse of cultists and mutants and even warships.

Regarding the campaign itself, I'd say it would depend largely on how your group looks like, meaning the player characters' classes as well as the roles they have chosen to occupy and the resources they have at their disposal. In all likelihood, I'd split it up into several phases (much like it was explained in the Vraks book), slowly shifting from covert ops and infiltration to armed conflict in the streets to all out warfare, with a small period of bloody tyranny and lots of rituals, sacrifices and desecration in-between the coup (where the PCs could become a "power behind the throne" of the Apostate Cardinal), and the arrival of Imperial forces attempting to recapture the planet, turning it into a warzone.

Ok, thank you for answered my questions. I know what i will do now. Concern my group, we have a Berzerker, a Thousand Son Sorcerer and a Unaligned Black Legionnarie Veteran of the Long War.

Sounds like a pretty balanced groups of CSM for a game such as this! The Sorcerer in particular could have a field day with the possibilities of infiltration and manipulation, and attempted "diplomacy" with Chaos-aligned rebels to coordinate insurgent efforts against the remnants of Imperial resistance. The Veteran on the other hand makes a perfect martial leader, like a cultist general of sorts. I'm not sure what the Berserker could do aside from combat - but then again it's not "wrong" to have a character limited mostly to this, as long as you make sure each of your players has their moments to shine in their chosen specialty.

Have fun and good luck with your game!

The loyalist fleet is always in orbit? If yes how many/powerfull is the fleet?

It most definitely is, and battlefleet Scarus will be enjoying void superiority, with dozens of cruisers and other capital ships and numerous troop transports and escorts providing logistics, orbit-to-surface bombardment, interdiction, patrols and the like.

Does this mean they control the entirety of the star system? Definitely not, and, while attempting to breach the blockade might be tricky, you can always count on warp magicks to give you a shortcut in form of portals, ritually opened by sacrificing disposable minions in a chaos ritual, or lightning strike tactics, picking off targets and rapidly retreating to asteroid belts.