Spectre appeared in Purgatory, when Constantine, Diggle, and Mia brought post-Lazarus Pit Oliver to try and restore his soul and bring him back to life as a whole man.
He showed up with glowing green eyes and introduced himself as Jim Corrigan, a former police officer who became something else after being “called to a higher purpose.”
First, though, let’s run down how Spectre joined the Arrow-verse in “Crisis on Infinite Earths” and why he’s so much more important than other Arrow-verse crossover newcomers, which included Lucifer’s Lucifer and Smallville’s Clark Kent with a surprising twist.