The final DLC for Rise of the Tomb Raider will be available on the 29th March. Cold Darkness Awakened will be available on both Steam and the Windows 10 Store. While the content is free for season pass owners, the price has not yet been revealed for non-owners.
Cold Darkness Awakened introduces a new Horde-style mode in which Lara Croft must fend of waves of enemies. Not much else is known about the content save the product page description: "A decommissioned Cold War research base has been breached, unleashing a deadly affliction from its Siberian slumber. As the illness seeps into the wilderness surrounding the base, Trinity soldiers transform into bloodthirsty killers who thrive on carnage. To prevent the affliction from spreading, Lara must rely on her wits, physical agility, and crafting abilities to overcome this new enemy and shut down the decommissioned base."
The "Complete Experience" trailer teases at some of the gameplay found in the Cold Darkness Awakened content.