The folks at Marvel have managed something incredibly unlikely: When audiences go to see their superhero movies, almost everyone stays in their seats through the closing credits. That's because it's become tradition for the company to use a secret end-credits scene to tease one of their next movies, and The Avengers is no different (in fact, the supersize blockbuster actually includes two secret scenes, one of which was shot after the movie had already screened for critics). This time, though, the tease may be a little obscure for moviegoers who aren't well versed in the Marvel canon.
Spoilers will follow, obviously.
At the end of the movie, partway through the closing credits, The Avengers spirits us away from earth into deep space, where we see two figures in conversation. Well, only one of them is actually talking: the Chitauri credited as "The Other," who had given Loki his alien army to use on Earth at the very beginning of the movie.
The Other relates Loki's defeat at the hands of the Avengers and says that to challenge them — and humanity — would be to court death. At that moment, the other figure half-turns and smiles, giving us a brief glimpse of who he is. But it's dark, and you're wearing 3-D glasses, and the person next to you is murmuring, "Is that ... Hellboy?"
It's actually the lantern-jawed Marvel villain Thanos. Devin Faraci at Badass Digest does a great deep-dive into Thanos as a comic-book character, but here's the short version: Thanos is an incredibly powerful interstellar being best known for the six-part Marvel series The Infinity Gauntlet, published in 1991. In it, the godlike Thanos gets his hand on the titular item, a golden glove studded with six Infinity Gems that gives him immortality and omnipotence. He uses that gift to slaughter half the universe, including tons of superheroes; when the remaining heroes travel into space to fight him, he kills all of them, too. Even the Avengers are creatively slaughtered, one by one. Iron Man gets straight-up decapitated! (Though things are eventually put right, and a lot of cosmic magic is involved.)
Suffice it to say, Thanos is a much, much bigger adversary than anything we've seen in a Marvel movie thus far, and a weirder one, too. The company has hinted at his big-screen introduction for a while: At Comic-Con in 2010 — the same year that Marvel rounded up all of the Avengers actors for the first time and presented them onstage — the Infinity Gauntlet prop was shown off for a brief five minutes on the show floor. Still, Black Widow is going to need a lot more than a measly pistol if she's going up against an interstellar Eternal who can murder half of humanity.
One other fun fact: That secret credits scene employs two actors who've become staples of director Joss Whedon, though both are disguised in layers of makeup, CG and motion-capture. The Other is played by Alexis Denisof, best known as Wesley on Angel, while Damion Poitier (credited only as "Man #1") plays Thanos and worked as a stunt double on twenty episodes of Whedon shows Angel and Dollhouse.