Check out the first promo for the ninth episode of the second season of Arrow, titled "Three Ghosts", which is the second part of tonights episode and also the mi-season finale. We don't know if Summer Glau is in this episode yet.
Here's the official description for this episode:
Arrow 2x09 "Three Ghosts" (Mid-Season Finale) - After a fight with Cyrus Gold AKA the Acolyte (guest star Graham Shiels), Oliver (Stephen Amell) is drugged and left for dead. Determined to cure him, Felicity (Emily Bett Rickards) makes a risky decision. The cure causes Oliver to hallucinate, and what he sees forces him to confront his past. Meanwhile, Barry (guest star Grant Gustin) continues to help Felicity, and their connection grows stronger. The Arrow asks Lance (Paul Blackthorne) to look into Gold, but when the stakeout on Gold goes bad, someone is killed. Roy (Colton Haynes) is captured and tortured by Brother Blood (guest star Kevin Alejandro). John Behring directed the episode with story by Greg Berlanti & Andrew Kresiberg and teleplay by Geoff Johns & Ben Sokolowski
Be sure to tune in to The CW next Wednesday at 8pm to watch the mid-season finale.