I know that in Oblivion Teinaava simply describes that they are "trained in the arts of stealth and assassination, and live a life in service to the mighty kingdom of Argonia. Any Shadowscale who lives to come of age is accepted into the Dark Brotherhood as a full member of the family."

I'm just wondering, for a possible RP I may do in the game, do they have any abilities that average assassins do not have (besides turning invisible via the shadow). Like do they extensively use certain magic skills like illusion or mysticism? Or are they just normal assassins that are more experienced from being trained from birth.

The only thing that's known about them was stated in Oblivion. I think there may have been one or two references to them in the Infernal City novel. I forgot.

I would imagine they're simply very skilled assassins. You could play around with their abilities to make it more creative - they ARE reptilian so perhaps they could blend into their surroundings more with a natural chameleon like ability. I'd definitely say since they undergo strict training from birth they'd be among the most skilled assassins in the DB regardless of special ability. Think about the hallmark qualities of what makes a good assassin and amplify them a bit - agility, speed, the ability to go undetected, the ability to blend in, the ability to use a bow or a short blade, and poisons, etc.

I imagine a Shadowscale as the type of assassin who usually doesn't fail. If you ever see one he'll be the last thing you see unless you're very skilled or very lucky.