Inquisitor

A half-elf steps from the shadows behind the mugger, seen only for an instant before he disappears, as the mugger collapses unconscious.

A dragonborn stands in front of a courtroom, laying out the case against the murderer on the other side of the courtroom.

A halfling sits on a rooftoop, aiming down with her crossbow at the gathering of cultists below, and smirks as she squeezes the trigger.

The Inquisitor is a Martial Archetype for the fighter class, specializing in delivering decisive blows to the wicked from the shadows they call home. Often, they are bounty hunters, clearing out the filth on the streets of their hometown. Some however, choose to expand out into the wider world, seeking to right wrongs on a much larger scale, or simply seeking to improve their skills and arsenal by broadening their experience. 

From the Shadows

Starting when you take this archetype at 3rd level, you gain proficiency in the Stealth skill, if you didn't already have it. 

In addition, you can use a bonus action to determine whether a creature is of an evil alignment, or is using some method to disguise their alignment, though you still cannot tell the alignment of such a creature.

Punish

Starting when you choose this archetype at 3rd level, your years of hunting the scum of the earth have resulted in the ability to dish out heavy damage with your blows. Whenever you hit a creature with an attack where you had advantage on the attack roll, or if the target is of an evil alignment, you deal an additional 1d6 force damage. This damage increases to 1d8 at level 10, 1d10 at level 15, and 1d12 at level 18.

Enhanced Critical

Your weapon attacks count as critical hits on a roll of 19-20. And yes, your Punish damage is also doubled.

Swift Strike

Fighting groups of evildoers often requires tactful use of guerrilla tactics. Starting at 10th level, whenever you kill an enemy with a weapon attack, you can use your reaction to Hide. In addition, if you miss with a ranged attack you made from stealth, your position is not revealed as normal.

Uncanny Critical

Your skills in battle are heightened to uncanny heights. Your weapon attacks now count as critical hits on a roll of 18-20.

All or Nothing

Sometimes, evil doesn't want to just lay down and die. In this case, you can usually remedy the situation by hitting it really, really hard.

Starting at level 18, you can use your action once per long rest to make a single weapon attack with advantage. On a hit, it counts as a critical hit, with all the damage that brings. On a critical hit, you deal your critical hit damage as normal, and the target needs to make a Constitution saving throw (8+[DEX or STR]+Proficiency) or be immediately reduced to 0 hit points. You can choose to leave the target alive, even if they fail this save.