It's Cho'Gall, the two-headed ogre magi of Warcraft fame. First announced at BlizzCon earlier this month, Cho'Gall's either "zany and innovative" or "a stupid gimmick" depending on who you talk to. Why? Because each of his heads is controlled by a different player—one for Cho, one for Gall.
The whole concept is similar in nature to StarCraft II's new Archon Mode, wherein two players control a single army in multiplayer. Except it's a MOBA, so controlling a shared pool of units becomes sharing a single unit. The end result is the same though: your friend realizes how bad you are at the game and stops talking to you.
Cho is the meatier character, boasting not just a suite of unique abilities but control over all movement. Gall is dragged along for the ride, trapped in an existential hell watching Cho screw up over and over again while trying to use his own abilities. It's the equivalent of jumping in a chopper in Battlefield and hoping the pilot doesn't immediately fly into a brick wall.
Get a good rapport going and some interesting opportunities open up, though. Cho, for instance, tosses a bomb that can only be detonated by the person playing Gall. Or not detonated, as the case may be.
If this all sounds interesting and you want to play Cho'Gall, well, good luck. Those who attended BlizzCon or bought the BlizzCon virtual ticket should already see him unlocked for free. The rest of you need to play and win two games as "the other head" with someone who owns Cho'Gall in order to obtain him permanently, so he'll spread like some sort of MOBA venereal disease.
Of course, Cho'Gall is only one small part (the most interesting part) of the most recent Heroes of the Storm patch. Also of note: Haunted Mines has been officially taken out of rotation for the moment, with Blizzard basically throwing up hands and saying "We've made too many maps."
You can read the rest of the latest patch notes here.