NCF – Extended list of Neutral creatures.

In Skyrim, completing the Thieves Guild questline granted a bonus to illusion magic (fear, invisibility, calm, frenzy spells).
They should be given invisibility, and the description should explain that they can infiltrate enemy lines using magical deception.

This is a must! In Ashan, assassins also use illusion magic and deception; the invisibility spell is called "Shroud."
The "steal ammunition" ability could also be a good fit.

An alternative upgrade could be related to organizing the Black Fang, while a regular upgrade could be related to a secret guild of assassins working for the rulers of the free cities. Their task is to discreetly eliminate spies, traitors, opponents of democracy, and simply those who are undesirable.