Aiyoku no Eustia is definitely worth a try. It is the best August game out there.
Recommendations huh? *Fanboy mode* Try Supreme Candy. This is by far my favorite eroge. Half light half dark atmosphere, witches, magicians, hidden main heroine, epic final scenario, pumpkins and candies - just everything you need! Plus, the protagonist is a damn bad-ass!
The main plot is not as mentally deep as Eustia, but is much more complicated and stunning. It's a surprise how they can mix lighthearted comedy and romance with brain-melting main plot. And I LOVE EM ALL!!!
I can rant 2 pages of essay right now, but you get the idea. Now go and pick up the eroge!