Permission to reproduce the image here was given by Arya, aka prop and costume maker Dashy.
Our sassy, smug-faced female Ryuk was performed by fictional character - says so on Facebook, must be true - and cosplayer Nerdalina Ballerina.
You can enjoy more of her Ryuk cosplay, with plenty of extra crossover pics with characters from other shows or gaming worlds, in her Oz Comic-Con 2015 photo album. They include another Ryuk Game of Thrones moment, as the shinigami crosses over universes to take the much contested Iron Throne.
Presumably having written off a number of Lannisters in her Death Note, in order to do so. Which can only save time for George R.R. Martin, who is very much prone to doing the same.