This is your reward for finishing RotTR's story mode. It will unlock just before the credits begin to roll.
Feel free to put it on adventurer (easy) if you're just here for achievements or want to enjoy the story, because unlike the XB1 version, you don't have to play on survivor difficulty.
Do note though that the enemies in the final chapters hit hard at close range, so have some healing items on hand and try to go for headshots.