The CW has now released the official description for the episode:
MATT RYAN REPRISES HIS ROLE AS JOHN CONSTANTINE ON ARROW — When things take a turn for the worse with Sara (guest star Caity Lotz), Oliver (Stephen Amell) calls in a favor from an old friend who deals in the mystical, John Constantine (guest star Matt Ryan). John Badham directed the episode written by Brian Ford Sullivan & Oscar Balderrama.
NBC recently cancelled Constantine much to the disastifaction of fans everywhere with a #SaveConstantine campaign launched which ended up with Matt Ryan appearing on Arrow. No other network has picked up the series. It's always possible Ryan could star in the Justice League Dark movie that is currently in development, but with WB keeping their movie and TV universes separate, that may be unlikely.
"Arrow" airs Wednesdays at 8pm ET starring Stephen Amell, Emily Bett Rickards, David Ramsey, Willa Holland, Katie Cassidy, Paul Blackthorne and John Barrowman.