Ok, here's my basic plan as it currently stands:
First, you will give him the interview you planned to give him in the first place. This will give you an opportunity to get a feel for his character and motivations. Drop a hint or two that you know he's not a normal human being in order to pique his interest further. Provided there are no worrisome issues during the interview, such as excessive levels of deception that make you think he might be a spy, tell him his application is accepted and that he's now a member of the Black Knights.
From there, tell him you want to have a conversation in private to discuss his role within your organization.
Once in the privacy of your office, you'll reveal a few interesting facts for free - you know that Diethard is not a normal man, to the extent that you know exactly what he has become, and that his very existence is Anathema to Britannia's rulers. You are in fact a similar existence, and happen to know quite a bit of the world that is hidden from the eyes of normal people, and you're willing to make him an offer.
He can swear an oath to you that he will not betray you and that he never reveals your identity without your permission. Breaking the oath will have dire consequences, but he'll know if something he is about to do will break it and so long as he keeps to the spirit of the oath he shouldn't have anything to worry about. The exchange for this is that you'll reveal the truths of the world - what he is, the world's real history, and even your identity - as well as bringing him into your inner circle. He'll have a front row seat to witness history being written, or could write it himself if he so desires.
- If he asks for an escape clause or notes that nothing in the oath prevents you from betraying him, suggest that either of you can end the deal by notifying the other party in a way that wouldn't in and of itself constitute betrayal (for example, in the middle of an operation where doing so would screw over the other party), after which neither of you can act against one another for a year and a day. He'll still be bound to keep your identity secret if that knowledge hasn't already become public, though. If he notes that a year is a long time, inform him that he has the potential to live for several thousand years, so in the grand scheme of things it really isn't. Negotiate further terms if needed, so long as any new terms are reasonable and can't likely be turned against you.
His alternatives to taking the deal are...
1. He's still a Black Knight. He just has to work his way through the ranks through merit and has to earn your trust the hard way. (you'll have a couple demons secretly watching him at all times)
2. He can walk away now, and the Black Knights let him leave unmolested. (you'll have some Anuhles follow him off site, after which they'll capture or kill him)
Stunt: <tbd>
Charm Use: <tbd>
I have a few ideas for the stunt. I'm thinking Oramus excellency would be best here, and possibly we could use the effect of Fourth Soul Devil Domain to let us pull out an apple in the middle of the conversation to theatrically represent us offering fruit from the Tree of Knowledge, as well as proving we have powers. Though I don't know if Lelouch has discovered that asset of his new power or not -
@Alexander89 would need to tell us that.
We could also use Shadow Spite Curse to reduce chances that Diethard would notice the lack of clauses in the oath we're offering that give him protection from us and outs, though he's probably got a good dice pool for Perception/Manipulation and whatever the other skill would be so there's no guarantee of that working entirely.
Any other thoughts?
