Valve have released the Frosthaven update which comes with a new co-op game mode where players must work together to protect the Ancient Tree.
Swept up in the zeal of unraveling and reshaping arcane energies, Grand Magus Rubick marches a mob into Frosthaven, intent on uprooting the original Frostivus tree to explore the mysterious font of power beneath it. To stop the misguided magus, five heroes must rise to the defense of the ancient tree—and fight to keep the Spirit of Frostivus alive!
The Rubick Arcana was also released, which comes with 115 stolen spell effects. Rubicks Immortal shoulder item from the recent Trove Carafe will also change colors to match what spell is being stolen. Frosthaven: A Frostivus Story will go from December 19th to January 3rd.
You can read all about the full update here: http://www.dota2.com/frosthaven