During a crossover episode on The Flash in season 2, Diggle made an appearance on the Scarlett Speedster’s show.
In the premiere of the fourth season, we were introduced to said mask, that was initially criticized as resembling Magneto’s helmet from the X-Men films.
Now The CW has released a sizzle reel, promoting all of the shows returning to the network this season, while introducing some new ones.
But at the end of the show’s third season, Oliver suggested that Diggle get a mask.