The achievements list and some files were also leaked shortly after. You can read from them that Eivor’s abilities and equipment will be increased with invisibility, teleportation arrows, ice powers and the ability to revive enemies. The trophies, on the other hand, can say something more about the plot itself and the world in which the action will take place. There are achievements for discovering all dwarven shelters, as well as for fully developing an entirely new weapon. So it can be expected that something is actually going on.
Unfortunately, the leak does not contain information about the release date of the add-on. Some believe that the DLC will appear next month, based on the premieres of previous expansions. However, it seems unlikely, as Ubisoft itself mentioned that the next additional content will not appear until 2022.