This is Russian cosplayer Xenia Shelkovskaya, doing one hell of an impersonation of English actress Natalie Dormer/Westeros Deal Wheeler Margaery Tyrell.
Just goes to show that while a costume certainly helps, any cosplayer looking to get really authentic can get as much from a wry smile and a turn of the head as a 1:1 replica of an outfit.
For anyone thinking this is just a case of her looking like Dorner in the first place, here’s some Witcher cosplay—that didn’t require that trademark smirk—where she looks nothing like her:
