Moving right along...

If you don't own the book:
I guess that I should anticipate the issue of someone reading this but not owning the proper book.

As most should know, the Shadowcaster is from the Tome of Magic.

What could I give away about the class safely...?

Basically, they use mysteries instead of spells, which start as arcane spells, turn into spell-like abilities, and then turn into supernatural abilities. They gain only 1 per level.
In addition, they gain supernatural fundamentals (think cantrips) as they level up, which they can eventually use at will.
Finally, they gain bonus feats, darkvision, and eventually ignore metabolic needs.

I should also list the classes shortcomings that led to this addition:
1. Mystery-using is reliant on both charisma and intelligence.
2. Shadowcasters gain only 1 mystery per level.
3. Mysteries are taken in sequential groups of 3 mysteries called paths. For example, you have to take the 1st level mystery of a path before you can take that path's 2nd level mystery.
5. There are only 60 mysteries in existance + 9 fundamentals. This list has been expanded officially once in a cityscape web enhancement, raising the count to 69 + 9 fundamentals.