At the tail end of the footage shown for Thor: Ragnarok at the Hall H panel at San Diego Comic-Con, there were a couple of villains, one already confirmed, and one a surprise. A fully rendered shot of Cate Blanchett was first as Hela, and she looked ripped out of the pages of the comics. It was unclear whether that was straight CGI or some mixture of a costume screentest and CGI, but she looked incredible.
However, that was quickly dwarfed for the 4,000 screaming fans when the final shot, full CGI, was displayed. The shot is early; they've literally been shooting this film for five days, and this is probably test footage. Hulk is roaring, jumping from some crazy height, ready to fight. As he dives to what's below him, everything around him is on fire - and that's when you realize, there's a face, and out of that face a louder scream than the Hulk's. It's Surtur, the fire demon of Muspelheim.
In the pages of Marvel Comics, Surtur is an immortal older than even Odin. His realm is completely fire, and he plays a key role in, you guessed it, the Ragnarok, the Norse apocalypse. In that story, he's destined to destroy Asgard using the Twilight Sword. If that sounded like a lot of nonsense to you, just know that he's about 10 stories tall, made of fire, and carries a mystical sword capable of destroying cities.
It looks like he'll be fighting that fight in Thor: Ragnarok, and while it looks like Thor and Hulk will fight at first (as seen earlier in the footage), Hulk won't let Surtur go unchecked.
Thor: Ragnarok is filming now for a November 2017 release.