“A range of content that will extend your single-player campaign, and also allow you to compete with friends. Three major releases will challenge you to survive the extreme conditions of Endurance Mode, explore a new tomb that houses an ancient terror in Baba Yaga: The Temple of the Witch, and combat waves of infected predators in Cold Darkness Awakened. In addition, a steady stream of outfits, weapons, and Expedition Cards beginning on Day 1 and continuing over four months will keep you customizing, expanding, and challenging your friends to new adventures.”
In my opinion, based on the names of the new modes, it seems likely that Baba Yaga: The Temple of the Witch will be its own mini story, similar to the DLC packs for games like Dragon Age Inquisition. The other modes will likely be survival modes that will likely have little or no story element. The season pass will be available on Nov. 10, which is the same day the game launches.
Microsoft recently announced an Xbox One bundle for Rise of the Tomb Raider. Besides the new game, the bundle will include the Tomb Raider Definitive Edition and will also come with a nice-sized 1 TB hard drive.