3) Season 2, Episode 16, “Suicide Squad” Despite wanting to focus on helping Oliver take down Slade Wilson, Diggle finds himself at A.R.G.U.S. headquarters with Amanda Waller.

Flash vs. Arrow (Arrow Season 3, Episode 8; The Flash Season 1, Episode 8, 2014) This was the first crossover event in the Arrowerse, a two-part event featuring The Flash and Arrow.

3) “Sacrifice” (Season 1, Episode 23) Arrow ‘s first finale doesn’t pull any punches, pitting Oliver against Malcolm Merlyn, aka the Dark Archer, who wants to level a portion of Starling City.