Deadly Silence is a passive Physical Skill that allows Scarlett to score extra damage on an enemy when she scores a surprise attack. Scarlett can gain surprise by sneaking up slowly and quietly behind an enemy (assuming the enemy is looking another direction.) Or, she can use The Passage to easily attack any enemy unaware.
Deadly Silence has only a single Level of Mastery. It is a passive skill that is invoked automatically whenever Scarlett attacks an opponent who was unaware of her presence. (After the first attack, combat proceeds normally.)