Seeing how you'd get spammable abilities via invocations, and specificity of options vie mysteries, you'd have one heck of a versatile character. You'd never be without something useful to do. Some of the time just spamming one trick over and over will be enough to overcome a challenge; at other times, you have more powerful, but more use-limited options to fall back on. I see this as tier 2, as it's probably easily on par with a Favored Soul or a Binder with online vestiges.
I like the flavor, I like the general idea. However, I think the normal Warlock is quite OK, a nice and balanced base class for most campaigns. Shadowcasters can be modified in different ways to take away from their suckage. Combining the two somehow seems a bit preposterous to me - you might as well go sticking all Ninja class features on a Warmage or something.