Katrina Law who was on Showtime's Spartacus is coming to Arrow to play a character related to the League of Assassins
She will show up as Nyssa Al Ghul, the daughter of Ra's and a Russian woman, in episode 13 Heir of the Demon. In the episode she will be sent to bring back Canary to the League of Assassins.
So my guess about Nyssa being in the show was correct but I got the wrong person, it isn't Summer Glau. I wonder though about Canary being called Beloved though and vainly hope there may be a relationship between the two. With this and Ollie getting a mask I am looking forward to the rest of this season.
Via Bleeding Cool