Lethal Assassin (5e Feat)
From D&D Wiki
You are a master in the art of killing. You have expertise with blades and you when you surprise an enemy can make a stealthed attack that has +2 to hit dealing +2d4 damage and does not break your stealth.
thrice per rest you can force the enemy to take a Con save at disadvantage (DC 10 + dexterity + stealth prof) or take 1d10+Dex mod poison damage per player level and be both slowed and weakened for 2d6 rounds.