The Assassin Class

Reese suggested in a comment that thief variants, like the Assassin or Acrobat, could be handled similar to the martial arts monk, with combat styles that work like those described in the martial arts post. That’s certainly possible, although traditionally assassins aren't modeled after monastic orders. They could be, but probably I’d just let assassins study martial arts styles that seemed useful for stealthy killing. For example, I've been thinking that the Thief default weapon would normally be the dagger or short sword (close-quarter weapons that can be easily concealed,) but the Assassin default weapon should probably be a crossbow, and they could study the Bow Master style and its secrets. Another option would be an Unarmed Master style with a variant Superheroic Secret based on the “quivering palm”.

I did an Assassin before, when I talked about modifying the Delving Deeper Thief to get an Assassin and a Monk. But I didn't do a formal write-up in my new class template.

Assassin

Concept: Professional trained in the secrets of killing. I haven’t played those “Assassin’s Creed” games (not that into video games, remember?) but I’m guessing that assassin is a lot like this, too.

Alignment: Neutral or Chaos, although a Lawful variant that focuses on supernatural evils might be interesting…

XP/HD: XP as Fighter, HD as Thief. Technically considered a Thief with extra abilities, hence the higher XP needed. Prime ability is lowest of Strength and Dexterity.

Weaponry and Armor: Weapons purchased when character is created.

Abilities: As Thief, but also has a Disguise ability (treat as Stealth/Hide in Shadows.)

Styles: Mastery of one of the three basic martial arts styles (see Martial Arts Monk.) Typically learned from a master assassin, rather than a monk; the Vow usually required is a cut of the assassin’s “action”, perhaps with obedience to an assassin’s order and a harsh code.