Well, if you want suggestions please tell us what does not fit you?

Are you playing a ranged assassin so death-cultist is out? Are you soley combat focused so interrogator is nothing for you?

Does your GM have a problem with power-level or balance (For Vindicare is commonly seen as way overpowered) and in combination with an untouchable it might get even more strange for such an individual would have been made into a cullexus, not a Vindicare TEMPLE assassin.

Well, now that you edited your first post I can give you more answers, next time just make a post and not a content edit, that makes it easier.

The first step for a Culexus is that your character has actually be educated by the officio assassinorum that normaly do not recruit mature people for children are much easier to turn into a weapon. Also the training in one of the temples of the assassinorum takes years, if not decades hence they prefer children for they finish their education when their body is in its best shape (in addition with some cybernetics/implants).

So, unless your GM makes up a quite strange story to allow a mature assassin with his own masters, own beliefes and motives enter such a temple you might get a problem here. And then your character has to spent years or magicaly learn in a month what others need a lifetime for (and those are already the best the imperium can offer)

The next problem is that your character is a blank and not a Pariah as it is required to become a Culexus. Depending on Fluff source there is no way to change here and again something has to be made up for that.

If your GM is willed to make all of these things happen I would take the Death Cultist Career, Prox it as a Culexus and ask for the Animus Speculum and Etherium or Psyk-Out Grenades from the Navis Primer. I would hardly suggest you to not take a custom career.

Well, thanks for your response, but I didn't actually edit my post at all?

I do appreciate that most temple assassins are trained from a young age, but the vindicare does have a specific transition package to support people being recruited mid career, and in the case of a group as rare as blanks/nulls/etc... I would imagine that the Culexus would do something similar.

As for what I'm looking for, I'm thinking of something a little more custom than just taking one ascended career and putting a different name on it. I'm wondering if anyone has done it before, or has ideas for the kind of skills a Culexus should have, or if there are any cool ideas for talents or traits, maybe an upgrade to the null effect or something.

Personally I would adhoc the background thing.

I would talk with my GM, and see if it was possible to retroactively make my char into a Culexus appentrice. In this way all of his previous experience would in fact have been his field training for the temple.

Perhaps he was brainwashed and drugged into believing a false background, so that he could not reveal details about the temple and the field training scheme.

Now that he has been deemed suitably fit to be a full assassin, the temple takes him back for his final training and indoctrination.

The joy of this method is that the GM can now gleefully saddle the char with a new host of allies and enemies in the temple, who all have opinions on this revolutionary training scheme.

For the system I would make a variant of the ascencion assassin.