WARNING: Mildish spoilers for Avengers 2: Age of Ultron (probably).
True fact: Marvel hasn't been clear yet on how the mutant children of Magneto will be explained in a movie that can't even use the word mutant.
So we've had to speculate on how Quicksilver and Scarlet Witch got their powers, with most of our hints coming from an end-scene in Captain America: The Winter Soldier.
This has led fans to believing they may be Inhumans, but Marvel has subtlety dropped the truth on us via a digital comic dropped for the movie's marketing.
The on-shot comic, "Avengers: Age of Ultron Prelude - This Scepter'd Isle," takes place between the first Avengers and Captain America: The Winter Soldier. It follows what exactly happened to Loki's scepter after he was defeated.
Well it's been revealed (sort of) that the scepter ended up with Baron Von Strucker, who we saw in the end scene during [Captain America: The Winter Soldier](movie:254973). Remember that scene with the twins? Turns out they received their powers from, you guessed it, Loki's scepter.
Strucker (and Hydra by default) used the scepter on his test subjects from Sokovia (a fictional city in Marvel's version of Europe) in an effort to unlock special powers. Hence, the twins, Quicksilver and Wanda (also known, of course, as Pietro and Wanda).
This is a clever twist on a big problem Marvel had to resolve. This "unlocking" certainly paves the way for making mutants and even [The Inhumans](movie:910490) fit within the stifled Marvel Cinematic Universe. And it will also make sense within the context of recent comics.
But what powers Loki's scepter, anyway? Most fans believe it is infused with a Terrigen Crystal due to its blue glow. In the comics, these crystals come from Terrigen Mist, which causes mutations and alters the abilities of the Inhumans.
Others believe an Infinity Stone or something residual from the Tesseract are connected to it. Maybe both?
At any rate, I'm glad they're using this within the story to provide worthy allies or even adversaries for Ultron, who will be a force of his own.
Avengers 2: Age of Ultron is set to release May 1, 2015.
What do you think powers Loki's scepter?