Ooooh! 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).
"A long-dormant evil stirs in Dark Reef prison, and four chosen defenders must band together on a secret mission to stop the fiendish Siltbreaker and his enthralled host of followers. To face this dire threat, all Battle Pass owners are invited to journey across vast landscapes filled with hordes of monsters and other lethal terrors" sez Valve over on the Dota 2 blog.
I've given the campaign a quick go and it reminds me of a cross between the Dota 2 tutorial where you're wandering around the forest, and some of the limited time event modes. There's an element of balancing what you spend on bolstering health and mana against what you need to budget for picking up stat-boosting items and utility for later wave defence bits. The section I played involved a LOT of punching wolves in forests. I will say that I found it peculiarly difficult to target critters with Witch Doctor so I'm interested to see if other people run into that or whether I'm just really rusty!
Playthroughs give chances to earn Artifacts which are equippable objects. It sounds like you earn some, then equip them to help you get a bit further next time and earn more and equip those and so on and so on.
Me paraphrasing is probably not as helpful as just quoting Valve's own explanation so here's what they say will happen as you play:
"As Act I opens, Siltbreaker awakens in the depths of Dark Reef, and the mysterious Conclave of the Brine calls upon you to halt this dark power’s ascent, starting with Siltbreaker's twisted creation—Rhyzik the Corruptor.
"As you battle through multiple play zones in search of Rhyzik, you'll earn completion stars based on your performance level. The first time you complete a zone with one or two stars will net you Battle Points rewards, and delivering a three-star zone performance will grant a treasure from Siltbreaker himself."
Battle Points help you level up the Battle Pass which is the digital doodad which gives you access to a bunch of seasonal cosmetics and rewards as well as all the wagering and supporting and collecting stuff relating to the big esports tournament, The International. Treasure is just Dota's version of loot chests.
"Each Siltbreaker Treasure includes a chance to receive an indescribably-rare Immortal Desert Sands Baby Roshan courier, and Battle Pass owners with Battle Level 165 or higher will receive double the treasures for each three-star zone they complete with the reward bonus active. You’ll also have the opportunity to unlock achievements that grant even more Battle Points."
So it sounds like Rhyzik is the first boss and Siltbreaker will wander in during Act 2.
The game mode is open to all who own the Battle Pass. The cheapest version of that is £7.99/$9.99. Alternatively, Blizzard are selling Warcraft 3 on the marketplace formerly known as Battle.net for £6.99/$9.99.