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.
AssassinConcept: 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.