Seems to me like a hybrid of vertain themes... from the posing I personally would see more classical portraits of kings an emperors in it. The flowing garb reminds me quite a lot of Louis XIV:
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Nice catch. But you're probably right, it's a melting-pot of some bohemian or polish aristocrats portrait we even don't know.
The name of the antagonist is quite a mouthful...
Dettlaff van der Eretein
Source: http://it.ign.com/m/ualone/111499/blog/i-miei-appunti-per-la-recensione-di-the-witcher-3-blood-and
Oh, I already see an easter egg.
The main antagonist is...
The concept artist Marta "Dettlaff".