Dota 2’s first co-op campaign arrives in Siltbreaker Act 1

The first part of Dota 2 [official site] co-op campaign, Siltbreaker, has arrived!

Act 1: The Sands of Fate is the game’s first co-op multiplayer campaign (unless you count crowding round a screen with a friend to take turns playing Warcraft 3 or something).

So it sounds like Rhyzik is the first boss and Siltbreaker will wander in during Act 2.