Here is my idea I want to use the deathwatch adventure Ark of Lost Souls. The party starts as Deathwatch marines but by the end of the adventure they will take the first steps to Black Crusade.

Question one Should I create them using the Deathwatch rule set then convert the survivors.

Question two Is it easy to do this type of conversion ?

Thanks

I don't have the Ark Of Lost Soul supplement, so I'm going to answer your question by another question.

How do you know your player will slighty go toward Chaos ? Can't they choose to remain "pure" and keep their faith in the Emperor in this campaign ?

In my opinion, you have two choices:

1- If your players know that they are going toward Chaos, just go for the Black Crusade character creation.

2- If they don't know about your plan, then use the Deathwatch character creation and then do the conversion when they are corrupted enough.

I just look upon the conversion rule and they don't seems to be that complicated, but I never tried them.

Since these are Deathwatch Marines, if they go over to Chaos, they must have accrued quite an amount of corruption points. Thus madness/quirks from geneseed degeneration. I'd use what's already in place in Deathwatch to simulate that, but a bit more customized and tailored to each character.

Other than that, the new Black Crusade characters should start at 0 Corruption (since they just went over), lower Infamy (they are, after all, faceless Deathwatch operatives) like 9+1d5, lose all the Cohesion stuff, but have quite a bit more starting xp to spend (with Deathwatch rules). Then when they go over, check for alignment (I doubt any would align so quickly, but you never know), and you could include hallucinations, visions and dreams linked to the God towards which each character leans the most in the adventure (I can't be more specific since I don't have that book). They should lose any fate points they had, their armour bonuses can stay, and you should replace their demeanours with prides and failings (failings especially could be linked to that specific adventure). I don't remember if there is any other stuff that doesn't cross over naturally.

I hope, btw, that your players agreed on the narrative (ie, they know that their Deathwatch characters will cross over to Black Crusade in that adventure and want to play their fall) or you might be in for a nasty surprise. If not, I hope you have a backup plan for when one of your characters stays pure and gets murdered by his erstwhile brothers (or the reverse). Narratively, the simplest way to achieve your goal is to goad (not force) them into committing atrocious acts in the name of their own survival until all pretense of nobility is gone. Then, have a third party (daemons? chaos marines? dreams promising power if they forsake the Emperor? xenos? heretical cult?) make them an offer they can't refuse. One that will tempt those more interested in their own survival than their vows, and that will get rebuked by the fanatics that truly believe their actions are guided by the Emperor. Cue either fratricide or you get what you want.