Warning: spoilers ahead for Episode 16 of Arrow Season 5, "Checkmate."
Arrow has been subjecting Oliver to a brand new breed of villain throughout Season 5 with Prometheus, and he's been a few steps ahead of Oliver all along. Team Arrow got a slight edge on Prometheus in "Checkmate" when they used the knowledge that he's really Adrian Chase to turn his wife against him, but Prometheus came out on top when he managed to kidnap Oliver as the Green Arrow. The good news for Oliver (and Season 6) is that Prometheus doesn't intend to kill Oliver. The bad news is that he intends to inflict so much suffering on Oliver that he's fundamentally changed as a person.
Prometheus does have a legitimate grievance against Oliver, insofar as Oliver did murder his dad way back when he first returned to Starling City, and it's clear that Prometheus has suffered a great deal. Throw in the fact that he was trained in archery and hand-to-hand combat by Talia al Ghul herself, and it's easy to imagine that he's had a rough life that he intends to make Oliver pay for. Given that he's a crazypants killer nowadays, we can bet that he has some nasty methods of torture in store for Oliver.
The big question is if Prometheus can possibly torture Oliver enough - mentally, physically, and/or emotionally - to fundamentally change him as a person. Oliver has already gone through personalized versions of hell over and over again throughout the last decade or so of his life. Every time he seems to hit rock bottom, he manages to bust through a layer of rock and dig even deeper. Sure, many of those rock bottoms are his own fault for poor communication, bad decision making, and/or thinking his with downstairs brain, but the fact remains that he's already been changed by crucible after crucible. Prometheus will have to get awfully creative if he's going to break Oliver anew.
The promo for next week's episode indicates that Oliver will face being carved up by knives, shot with arrows, waterboarded, and forced to face the reality that his decisions have caused great pain for people he cares about. Apparently Prometheus didn't tune in to Seasons 1 - 4 before coming up with his torture regimen, because Oliver has already been through all of those things. Of course, the episode does promise to be totally torturous for those of us who have spent the last season or so trying to forget that the baby mama drama ever happened; Prometheus will be bringing up William as part of his attempts to torment Oliver into becoming a brand new person.
We'll have to wait and see what happens. Oliver has survived arguably worse than Prometheus over the past nine years of his life, but it looks like Prometheus will be hitting him in all his sensitive spots in quick succession. While I'll admit it's possible that Prometheus will succeed in changing Oliver for a span of episodes, I'm inclined to believe that he's endured enough that Prometheus can't irreversibly change him in a few torture sessions. Maybe it's just that I don't want to have to watch our hero revert back into a ruthless killer after spending four and a half years watching him develop. I'm guessing that next week could be a dark, twisted, and perhaps even intriguing hour of TV.