Warning: spoilers ahead for the Arrow Season 5 midseason finale, called "What We Leave Behind."
Arrow has spent most of Season 5 building up the mystery of the new big bad Prometheus, and fans have been speculating ever since the premiere about the man behind the mask. There have been plenty of candidates, ranging from newbies like Felicity's boyfriend Billy to a crazypants resurrected Tommy Merlyn. We even had a fake-out that Quentin might have been Prometheus. The midseason finale finally revealed Prometheus' identity, and he just so happens to be somebody who lost his dad to the Hood back in Season 1 and went more than a little bonkers in the aftermath.
Prometheus kicked off the action of "What We Leave Behind" by using the information handed over to him by Evelyn to target Curtis. He hit Curtis with a throwing star that turned out to be laced with a drug called dycloseral, which was first put into production years ago by Justin Claybourne of Claybourne Pharmaceuticals as a very pricy way to battle tuberculosis. Back in his first year of vigilantism, Oliver discovered Claybourne Pharmaceuticals when TB broke out in a struggling area of Star(ling) City, and he ultimately learned that Claybourne had deliberately released the bacteria as a bioweapon. Since Claybourne's name was on his list and was clearly a pretty bad guy, Oliver went to pay him a visit, and Claybourne ended up dead, leaving his illegitimate son to spend the next four years marinating on his anger at the Hood who killed his father.
His plan for vengeance finally kicked into gear after four years of waiting when he lured Oliver to the building that had once been Claybourne Pharmaceuticals. Prometheus and Oliver battled it out until Oliver gained the upper hand. Our hero drew his bow and shot Prometheus dead with arrows in his chest. Huzzah!
Or so it seemed, anyway. When Oliver unmasked the dead man in the Prometheus mask, he discovered that he had actually killed Felicity's poor unfortunate boyfriend. Billy had been working the Prometheus case when he was captured, abducted, and dressed up like the Season 5 supervillain to trick Oliver into offing one of the few non-crooked cops in Star City. Prometheus escaped to scheme another day while Oliver had to return to the Arrow Bunker and break the bad news to Felicity. The grand Arrow tradition of terrible things happening at Christmastime continues unbroken.
In many ways, the Season 5 midseason finale with the Prometheus reveal was quite similar to the Season 1 midseason finale with the Dark Archer/Malcolm Merlyn reveal. The flashbacks of "What We Leave Behind" were even to Oliver's activities as the Hood back in the early days. Oliver and Diggle were back in the foundry, Felicity was rocking a low ponytail at Queen Consolidated, and the Hood was as murder-happy as he ever was. Ah, memories.
Honestly, the entire episode was set up in a way that would have made it positively perfect if Tommy Merlyn was revealed as Prometheus. Malcolm Merlyn was the bad guy in the Season 1 midseason finale; his tragically warped son could have been the bad guy in the Season 5 midseason finale. Instead, we got a random guy retconned into existence via flashbacks that didn't even 100% successfully de-age Stephen Amell to his Season 1 look.
Personally, I'm glad that the Prometheus reveal happened in the midseason finale, but I'm pretty bummed that the man behind the mask is somebody brand new. Prometheus could have been so much more impactful as Oliver's nemesis in Season 5 if he was somebody that we knew from Season 1. Legacy is the theme of Arrow this year, and it would have been nice to have an actual legacy character come back as the big bad instead of the illegitimate son of a man we'd never heard of before. Even if Prometheus couldn't be Tommy, he could have been somebody else we'd seen before, and it could have made for much more of a "Holy shit!" moment in the midseason finale.