In combat, you can sometimes catch your foes off guard with a taunt or a look that shakes their confidence to its very core.
When an enemy attacks you for the first time in melee combat, you may spend expertise dice to make an intimidate check against them. This roll is a contest. Roll all expertise dice you spend, but add only the highest to your skill roll for this contest. If you win, add your Charisma modifier to your AC for attacks by this opponent for the rest of combat. If your total for this contest is more than double your opponent's, that foe cannot attack you this turn.
You use cunning and guile to deliver a deadly attack against an unsuspecting foe.
When you make a melee attack against a foe and have advantage, or it is in reach of a creature friendly to you, you may spend expertise dice to make the attack a deadly blow. Roll all the expertise dice you spend and add only the highest to your Dexterity modifer. The result is subtracted from the d20 roll required for this attack to be a critical strike.
(Example: you roll 2d6 expertise dice and get a 2 and a 5. Your Dexterity modifer is +3. Your sneak attack total is 8, so this attack will critical on a roll of 12 or better)
You are adept at making yourself difficult to spot. In the confusion of battle you can use this to your advantage.
In any environment where you could hide, or if you are in reach of three or more creatures, you make spend your action to make a Sneak roll. Any creature that fails its perception check will still be able to see you, but will have disadvantage on attacks of opportunity against you until your next turn. You may spend one additional expertise die to take the Dodge action when using this manuever.
Level 3: Vital Strike
You have studied the biology and vital systems of various species. When you successfully assess a foe's vulnerable points, it makes you even more deadly in combat.
You gain training in the Knowledge: Anatomy skill. When you make an attack with a piercing or slashing weapon you may roll this skill against your target's AC. If you succede, add your Intelligence bonus to your damage for the attack, in addition to other bonuses. If you have advantage or disadvantage on the attack it applies to this roll as well. If the target is of your own race you gain a +2 to this check. For attacks against certain magical beings or constructs the DM may rule that a different knowledge skill applies instead.
Level 5: Artful Dodge
You have learned to use your sure-footing and nimbleness to your advantage, putting your foes off balance. When you successfully avoid a melee attack you may make a Tumble or Balance check, with DC equal to the attack roll total. If you succede, until your next turn melee attacks against you have disadvantage.
Level 6: Deadly Grace
You use your skills so well in combat that luck tends to find you when you need it. Once per day, when you roll a natural 20 on a trained skill in combat, you may roll again (including any expertise dice) and use the second result. If you succeed you may apply the natural 20 to an attack roll later in the same combat. This ability increases to twice per day at level 12 and three times per day at level 18.