The Prophet's Legacy pack is available now for Rise of the Tomb Raider on both Xbox One and Xbox 360, and is part of the season pass or for purchase at $2.99.
Includes the "Immortal Guardian" outfit that reduces Lara's damage from Greek Fire, the "Piercing Truth" dagger and a card pack with two guaranteed rare cards and three other cards that have an increased chance at being rare.
Source:
Official Tomb Raider Blog,
Xbox