Sylvanas Windrunner, Queen of
ALL ABS ALL THE TIME the Forsaken, was a known quantity in that she's been a confirmed HotS character since BlizzCon 2014 - I thought she'd been mentioned at a previous Blizzcon too, back when it was called Blizzard DOTA (I'm trying to find the bit in the panel video at the moment) - but there hadn't been any detail on how she plays until the developer's 2015 PAX East panel.
If the nitty gritty of what she does isn't important to you, the bottom line is that she's a ranged DPS-y character, good for sieging and with an option to subvert enemy pushes. For the more detail-oriented there's the following:
Her Q ability is Withering Fire - it's rapid arrow fire for burst damage which, looking at the video, doesn't need you to be actively firing and targeting to use. Instead it looks more like a mini version of Valla's strafe talent.
W is Shadow Dagger which does light damage to its initial target, then damage over time which spreads to nearby units, so good for wave clearance and against teamfight-y enemies.
Bound to E is Haunting Wave which sends out a wave of damaging banshees which Sylvanas can teleport to by hitting E again. It's similar to Illusory Orb - an ability from Dota 2's Puck and looks like it offer similar repositioning, escape and chase options.
Her heroic abilities are Wailing Arrow - an explosive arrow that deals area of effect damage and silences enemy heroes for a few seconds - and Possession which forces an enemy minion or mercenary to fight for you. The cooldown on the latter is linked to the strength of what you choose to possess.