From the 2014 PHB:
"Monks of the Way of Shadow follow a tradition that values stealth and subterfuge. These monks might be called ninjas or shadowdancers, and they serve as spies and assassins. Sometimes the members of a ninja monastery are family members, forming a clan sworn to secrecy about their arts and missions. Other monasteries are more like thievesā guilds, hiring out their services to nobles, rich merchants, or anyone else who can pay their fees. Regardless of their methods, the heads of these monasteries expect the unquestioning obedience of their students."
They are such good ninjas that nobody even knows they are explicitly and intentionally in there!
(I know people might mean, "I know about that, but I'm looking for something else," but to me it always feels like "If you wanted to add an eldritch knight to 5e, how would you do so?")