Embrace Death and Wield Ghostly Power
Some Rogues traverse the veil between life and death, shepherding opponents to the grave and slipping through the world as undetectable as a spirit. In these pursuits, a Rogue might discover a mystical connection to death itself. Such an individual becomes immersed in negative energy, infusing their strikes with deathly energy and stealing knowledge from souls who have passed on. Thieves' guilds value Rogues of this persuasion as highly effective information gatherers and spies.
Wails from the Grave
Immediately after you deal Sneak Attack damage to a creature on your turn, you can target a second creature that you can see within 30 feet of the first creature. Roll half the number of Sneak Attack damage dice for your level (round up), and the second creature takes Necrotic damage equal to the roll's total as wails of the dead sound around it.
You can use this feature a number of times equal to your Dexterity modifier (minimum of once), and you regain all expended uses when you finish a Long Rest.
Whispers of the Dead
Whenever you finish a Short or Long Rest, you can choose one skill or tool proficiency that you lack and gain it, as a ghostly presence shares its knowledge with you. You lose this proficiency when you use this benefit again to choose a different proficiency.