Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have Advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.
For reference, creatures that are Surprised can't move or take actions on their first turn of combat if they fail to notice approaching threats. This is done by comparing the Dexterity (stealth) checks of those hiding to the Wisdom (perception) checks of those not at the start of combat.
Starting at the 17th level, you become a master of instant death. When you attack and hit a creature that is surprised, it must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus.) On a failed save, double the damage of your attack against the creature.
At the 13th level, you gain the ability to unerringly mimic another person's speech, writing, and behavior. You must spend at least three hours studying these three components of the person's behavior, listening to speech, examining handwriting, and observing mannerisms. Your ruse is indiscernible to the casual observer. If a wary creature suspects something is amiss, you have advantage on any Charisma (deception) check you make to avoid detection.
You know how to strike subtly and exploit a foe’s distraction. Once per turn, you can deal extra 1d6 damage to one creature you hit with an attack if you have Advantage on the attack roll. The attack must use a finesse or a ranged weapon. As you gain levels, the amount of damage increases, as shown on the level table.
You don't need Advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don't have Disadvantage on the attack roll.
Rogues typically only get one Sneak Attack per round of combat. However if some circumstance or feature allows them to react during another creature's turn, such as an Attack of Opportunity the Rogue may get a second or third Sneak Attack during a single round of combat.
Starting at 5th level, when an attacker that you can see hits you with an Attack, you can use your Reaction to halve the attack's damage against you.
1)Only damage resulting from attack rolls such as melee attacks, ranged attacks, or spells with attack rolls (i.e. firebolt, guiding bolt) qualify for Uncanny Dodge.
1)Due to the restriction of using your Reaction, only one attack per round can qualify for Uncanny Dodge.
This action can be used only to take the Dash, Disengage, or Hide action.