Ever since fans saw John Constantine give Oliver Queen the tattoo off his arm in Arrow, they've wanted to see it in action. The scene obviously seemed to be setting up the confrontation between Oliver and flashback adversary Baron Reiter, but there has been little indication of just how the tattoo would come into play.
Now, a little more light has been shed on the situation. Perhaps more importantly, there's also been a hint of what the tattoo's limitations may be.
Executive producer Wendy Mericle recently commented on the tattoo, saying that "Basically Constantine says to him that this is insurance against Reiter, and we’re going to deliver on that promise entirely." She goes on to explain that it's going to come into play when Oliver has no other options:
"It’s going to help Oliver when he’s in a very dark place and time when he has no other way out. It’s going to be the thing that pulls him out of a very dark spot and literally saves him."
Some fans have speculated that tattoo would also help in the battle against Damien Darhk, but it may not prove terribly effective against the season 4 Big Bad. Mericle explained, "the mysticism that Reiter is practicing may or may not sync up with what Damien Darhk is doing," hinting that the tattoo is tied to a specific sort of magic and doesn't provide universal protection.
Of course, all of this assumes that the magic of the tattoo isn't used up in the battle with Reiter in the first place. While the tattoo has been present since the beginning of the series, there's been no indication prior to Constantine's appearance in 'Haunted' that it had any more significance than his other tattoos. While this is obviously because Constantine was tied up with NBC for a while and Arrow producers likely weren't planning season 4 out that far in advance, it's possible that by the time of the modern storyline, the tattoos have been reduced to little more than ink.
Fans obviously have different ideas about how the tattoo will play out and whether it will retain any significance in the present. All that's known is it will definitely play a role in the flashbacks, and will give Oliver a way out when Reiter would otherwise have him beaten. Regardless of theories, we should find out exactly how it works soon.
Arrow airs on Wednesdays at 8/7 Central on the CW.