I've always seen the Sidar similar to those spirits that fought for Aragorn in LotR, only now they're a civilization and not a band of cursed spirits. I don't think the 'ninja' concept would be best for the Sidar because when you examine the lore, it doesn't fit like the other 'ninja' types.
Shades, from what I know of mythology and fictional theory, aren't the types to never be seen or assassinate the regular people. Shades are the people who, after serving their civilization, undergo the rite that effectively eliminates their body leaving their soul intact. They then choose to exist in this plane despite their ability to exist outside of it, and continue their service for their people.
Gameplay wise, their high level units an turn to Shades and become specialists in cities. They also have limited invisibility abilities, but they don't really emphasize that. They are typically more defensive, rather than influencing territories outside their own.
Most of you already know this. Thing is, what should really stand out about the Sidar? I think it should be the original concept of the Shade, not only as specialists, but being able to exist in a different plane of existence.
Gameplay wise, it could be something along the lines of an Immortal type ability, but only if they're killed using physical means. Because they're not entirely of the physical world, they could come back to, say, the capitol, only perhaps with a small penalty, like an experience drop?
Also, since they have to be centered around their major cities, probably some UBs that will fit the lore and mythology of the Shades and just how they maintain a civilization within one plane of existence while they can exist outside of it as well.
Just some thoughts I had. I hope this isn't confusing to anybody, I sometimes confuse myself when I'm conceptualizing.
