A couple of questions:

- Do you know which character this is (i.e. is it a specific character, or a random one). Is he already coded in the ds (as a child)? Do you know the father?
- What type of trait is it? Is it something that you'd want to die out with the character, or could it be passed on to his sons?

Uniqe traits are not really possible, only in a roundabout way, really. Still, there are some ways it can be done, provided information is already known. That's why I had to make Steward of Gondor an ancillary :(

Another way is to give a trait that's automatically given to all new generals, then this trait would give the Ancillary, which is unique worldwide, and this ancillary would trigger more unique traits.
I could not get the FactionwideAncillaryExists & WorldwideAncillaryExists conditions to work in the traits files in BI 1.6.