The "Spartacus" star joined the show earlier this season as Slade Wilson, who eventually becomes the villainous Deathstroke in the DC Comics' Green Arrow. It's unclear whether his TV counterpart will follow a similar path.
The character on the series has so far been presented as an ally and mentor to Oliver Queen (Stephen Amell) during flashback scenes to their time on an island prison.
Bennett joins recently promoted cast members Colton Haynes (Roy Harper) and Emily Bett Rickards (Felicity Smoak) as series regulars for "Arrow's" second season, which is expected to premiere in the fall. The show was renewed last month after becoming the network's highest-rated series with an average of 4.3 million viewers.
The New Zealand-born actor is currently wrapping the third and final season of Starz's "Spartacus: War of the Damned," where he plays Crixus. He also appears in Peter Jackson's "The Hobbit" trilogy as Azog the Defiler.
"Arrow" continues Wednesdays at 8/7 p.m. CT on The CW, while the series finale of "Spartacus" airs on Friday, April 12 on Starz.