It is the next Magic set, Zendikar Rising. Tomorrow is the first day of previews, where they will be unveiling the set.
However, the tweet implies that there will be something for D&D players in the stream, perhaps some sort of tie-in.
Edit: I should mention that current rumor in the MtG community is that the set will be "class tribal" with a focus on the traditional D&D classes (fighter, rogue, wizard, cleric). I.e. all the intelligent creatures will have one of those classes, and there will be cards like "all rogues get +1/+1".