Game of Thrones season four is welcoming a new cast member when it returns, and we're sure pop culture buffs will nerd out over this one: Sherlock co-creator and star Mark Gatiss has been cast on the HBO fantasy hit as...

We don't know! Yes, Gatiss' role is being kept top-secret, much like Iwan Rheon's role prior to season three. (He was later revealed to be Ramsey Bolton).

In addition to his time opposite Benedict Cumberbatch on BBC's beloved Sherlock as Holmes' brother Mycroft, Gatiss has written plays and Doctor Who novels and acted in three episodes of the cult classic TV series.

Sherlock, Doctor Who and Game of Thrones? That's like the geek Triple Crown! EW.comfirst reported Gatiss' casting.