Last year, an early photograph from the upcoming Wonder Woman film depicted Princess Diana stealing a sword from her home on Themyscyria. As for why she stole the weapon, director Patty Jenkins explained, "This is her moment. She’s heard all the terrible things about man’s land. But she’s also heard that mankind is in need and under duress. This is her great moment to make the decision to be the one to try and save them.”
What's most interesting about this scene is that the sword Diana is stealing is not a classic part of the Wonder Woman mythos, like so many other aspects of the film. No, the so-called "God-Killer" sword actually debuted just two years ago, and it wasn't even in an issue of Wonder Woman! How did a sword from an entirely different comic book end up as part of Wonder Woman's movie debut?
The first appearance of the God-Killer Sword was in Deathstroke #7 (by Tony Daniel, James Bonny and Sandu Florea)...
In the issue, we learned that Slade "Deathstroke" Wilson had been hired by the Greek god Hephaestus (the god of many things, but perhaps most famously known for being the god of blacksmiths and thus, also one of of the most famous sword makers in mythology) to kill the Titan Lapetus, the God of Mortality, who was trapped in the depths of the Tartarus, but imprisonment was determined to be too good for Lapetus, so that's how both Deathstroke and the God-Killer Sword came into play...
The Wonder Woman mythos came into play right away, as Deathstroke had to get to the Tartarus through an entrance on Paradise Island. Luckily, the God-Killer Sword works as a sort of tracking device for its target, so it dragged Deathstroke to the Tartarus. Another feature of the sword is that it allowed Hephaestus to communicate with Deathstroke telepathically. The problem emerge when they got to the Tartarus and discovered that Lapetus was no longer there. They found a statue of him instead. The sword somehow pushed Deathstroke into using it it to destroy the statue. After doing so, he soon found himself face to face with Wonder Woman, who let him know that he had inadvertently unleashed a deadly force upon the world by destroying the statue!
As it turned out, the statue contained the life force of Lapetus. While Deathstroke had screwed up, the confrontation with Wonder Woman in Deathstroke #8 was perfect for demonstrating just how powerful the God-Killer Sword was! The sword responded to Deathstroke's thoughts but also acted independently in both defending Deathstroke from attacks but even going on the offensive against Wonder Woman itself. In addition, the sword can change its shape to fit whatever weapon is best needed for the mission at hand. Seeing that the sword only worked for Deathstroke (and after her lasso of truth showed that Deathstroke was legitimately trying to help) wonder Woman agreed to team up with Deathstroke (mostly so she would have the sword on her side) to take on the now once again corporeal Lapetus.
After a brief detour to a hell dimension (sent there by Lapetus), Deathstroke and Wonder Woman took on Lapetus alongside Wonder Woman's Amazon sisters. They also got an assist by Superman, who showed up to assist Wonder Woman (who he was dating at the time). He instantly blamed Deathstroke. This gave another chance to show off the power of the sword, as in Deathstroke #9 (by Tony Daniel, James Bonny, Eduardo Pansica and Sandu Florea), Deathstroke uses the sword to fight Superman it does shockingly well, as it appears to be able to take whatever energy that is used against it (whether it be heat energy or kinetic energy) and send it back to its opponent two-fold! So when Superman attacked, it just double his attack right back at him!
The stronger Superman attacked, the more powerful the sword responded. So yes, the God-Killer Sword is tough enough that it could even be used to kill Superman!
Wonder Woman broke up the fight between Superman and Deathstroke and since Superman was vulnerable to Lapetus' magic-based powers, it came down to Deathstroke to kill Lapetus in Deathstroke #10 (by Tony Daniel, James Bonny and Tyler Kirkham). Luckily, he discovered yet another power of the sword. It can draw out the memories of its opponents and feed them into the mind of the sword-wielder. In addition, it can serve as a sort of vacuum for spirits, as Deathstroke uses it to draw out the spirit of Lapetus' sister, whose life force had been powering Lapetus' magical staff...
Deathstroke then successfully killed Lapetus. Once he did so, though, the mission was over and he had to give the sword back (and, of course, a blood sacrifice over killing a Titan that Hephaestus failed to mention. Deathstroke paid it off by tearing out his own eye, which he had only recently been healed after years of being with just one eye. Presumably he figured it was easy to go back to just one).
Now, the question, of course, is why is the sword used by Wonder Woman in her upcoming film? First off, since the sword was created by Hephaestus, it is a simple enough tweak to say that he created it for the Amazons. More specifically, though, while it appears as though the sword in the film will be different from the comic in that it is likely will not change shape or have a will of its own, the whole "ability to kill gods" aspect is probably still intact, and that obviously could come in handy if Wonder Woman finds herself fighting against a certain god of war named Ares.
If that is the case, then this would be a clever case of the film's screenwriters, Allan Heinberg, Jason Fuchs and Zack Snyder, turning to the comic books to come up with an "in continuity" solution to a challenge within their film. In addition, a character as cool as Wonder Woman probably should have a very special sword, even if the comic book version of the sword has not been used by Wonder Woman (even there, it is probably only a matter of when the sword ends up in her hands in the comics).