This episode is one to the Season 1 flashbacks, with Oliver confronting the villain and mentor from that storyline. In the Season 4 flashbacks, Constantine explained that Lian Yu is a nexus of mystic energy. Now, those energies are spiking, causing resurrections. See more
The name of the island they found me on is Lian Yu. It's Mandarin for purgatory. Now I understand why. The memories, the pain, I've never escaped them. They're always with me, a reminder of what I had to endure, with only one thought, one goal -survive. To save the multiverse, I must become something else. To do that, I have to return where my journey began - Purgatory.