Every month, Unearthed Arcana gives players a chance to add some variety to their campaigns in a semi-official way. The rules aren't officially-official until they're later published, which most of them will be, but in the meantime, you can use them for your games. Besides, as I've said before, all RPG material is *pirate voice* more like guidelines, anyway */pirate voice* to begin with.
This particular article focuses on the more demonic portions of D&D. No, I don't mean those "D&D is Satanic!" videos from the 80s. I mean things like tieflings. In particular, different variants on tieflings to make them have more diabolic or demonic heritages. There's also new spells with which you can bring demons and such to do your bidding for you (alignment changes ahoy!).