After a close encounter with death Oliver has hallucinations of Shado and Slade, forcing him to confront a fateful encounter with Ivo on the island. Sebastian Blood plans to mass-produce his serum, and the police find Cyrus Gold to be more than they can handle.

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This episode is the second of a two-parter featuring the character of Barry Allen, a.k.a. "the Flash" (played by actor Grant Gustin). He'll return near the end of the season, in episode 20.
From The CW:

OLIVER IS VISITED BY GHOSTS FROM THE PAST - 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).