With The International 4's prize pool quickly approaching $8,000,000 Valve continues to release Dota 2 prizes for anyone who bought this years Compendium for the tournament. The newest update adds a few new prizes like the evolving battle booster to speed up your leveling, as well as the Immortal Strongbox which holds eleven brand new Immortal treasures. You get one of these strongboxes just for having the compendium, and you will get another for every 10 compendium levels you achieve. This box follows the rules of a normal treasure, meaning you won't get any duplicate treasures for opening multiple.
I just opened my Immortal Strongbox and got a pretty awesome item for Death Prophet. I really wanted the Storm Spirit hat, but I'll probably be getting another strongbox after a few more Compendium levels. Hopefully I can core one of the special treasures on my next one.