The Old Black Magic - I'm a big fan of this one. The first thing the article does is redesign tieflings, breaking them down subraces. One is the infernal, which is PHB version of the tiefling. The second subrace is the Abyssal, which includes randomized abilities. This one was exciting for those of us that miss the classic Planescape style tiefling. The second part of the article features demon summoning spells which can lend a dark aspect to a character or be used to power up enemy casters. The important thing about allowing characters to use this material is to think about how the common folk of Barovia will act. It can lead to some great roleplaying opportunities.
Gothic Characters - Just released today, this is Mike Mearls Ravenloft character options for those that were upset by the lack of them in Curse of Strahd. First we are presented with the revenant subrace. It includes the revenant subrace for humans, dragonborn, and tieflings. The subrace can also be used to bring characters back from the dead, which is a nice option. After revenant, we are presented with two character archetypes. The first is the Monster Hunter fighter archetype. It gains some basic ritual magic, the ability to speak an outsider language, and superiority dice like some other fighter archetypes. The second new archetype is the Inquisitive Rogue. This is a really cool Rudolph Van Richten style rogue archetype. They grant characters an eye for detail that can be devastating to foes.
If you're curious about the other Unearthed Arcana posts you can find them on my featured, D&D For Those Just Dropping By Post, which is updated with each new Unearthed Arcana article.