One of the biggest obstacles the "Avengers" sequel had to overcome was introducing, and constantly referring to two new characters, the Scarlet Witch and Quicksilver.

In the comic world, fans know the super-powered brother and sister duo as X-Men. Scarlet Witch and Quicksilver, aka Wanda and Pietro Maximoff, are the children of Magneto, who has been played on screen by Ian McKellen and Michael Fassbender, and are one of many characters referred to as mutants for having a gene which allows them super abilities.

Naturally, you would expect to see the two in an "X-Men" film since Marvel licensed 20th Century Fox the rights to use those characters from Marvel in 1993. 

Last year, we sort of did. Evan Peters played a young, rambunctious version of Quicksilver, who quickly became a fan favorite. His sister, Wanda, was hinted at in a brief deleted scene. 

Since the two characters have been a part of the Avengers team throughout the years, however, the two are also allowed to pop up in Disney's Marvel movie "The Avengers: Age of Ultron." 

Disney and 20th Century Fox's shared rights over the Marvel brother and sister allows the characters to appear in both an X-Men and Avengers film played by two different sets of actors.

If that doesn't already sound slightly confusing, here's where things get trickier.

While Quicksilver and the Scarlet Witch are allowed to appear on screen as "Avengers," they're not allowed to be referred to as mutants, a term which is exclusive to the X-Men. They're also not allowed to have any connection to their famous father, Magneto.

Disney, which owns Marvel publishing, has recently retconned the siblings' comic book origin story to cut their tie with Magneto and the mutants, presumably as a way to promote its own movies ahead of Fox's.

"Age of Ultron" also establishes its own mutant-free origin story.

One thought was that they may be referred to as Inhumans, a group of genetically-altered humans created to make an advanced race.

Marvel head Kevin Feige has said, however, there would be no connection between the twins and the Inhumans, who will be getting their own movie in November 2018. 

Instead, the "Avengers" sequel gives the twins a new origin story.

Some minor spoilers follow.

In "Ultron," Pietro and Wanda Maximoff are of Eastern European descent. To rid of any connection to the X-Men, it's revealed that at the age of 10, their parents were killed in a bombing.

The twins later join up with terrorist organization H.Y.D.R.A. to undergo some voluntary experiments and end up gaining super abilities. Pietro, aka Quicksilver, is lightening fast, while Wanda, the Scarlet Witch, has telekinetic abilities.

Instead of being called mutants, the duo, and anyone with special powers, is referred to as enhanced. 

You can see the Scarlet Witch and Quicksilver in "The Avengers: Age of Ultron" out May 1 in the US.

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SEE ALSO: Our review of "Avengers: Age of Ultron