Master Skull is the boss of Lara's Nightmare DLC in Rise of the Tomb Raider.
Story
Lara's uncle Atlas DeMornay curses Croft Manor in a final effort to drive her from the house.
The Master Skull controls all the undead creatures swarming the manor. It remains hidden within the foyer of Croft Manor.
Lara Croft must destroy three Skull of Rage to summon Master Skull. Master Skull has an ability to raise a shield, while the infected swarm Lara, killing all the infected disables it's shield. Killing Master Skull ends the expedition.
Strategy
The Master Skull must be shot until it raises it's shield, upon which it will summon more infected to attack Lara. Lara must dispatch the undead in the area and the skull will lower it's shield, this process is repeated twice.
