While the Epic Game Store has been divisive, you can't say that it hasn't been generous with the giveaways. Originally, the store gave away one game every two weeks, but that changed to one game every week - recent weeks have had two games each week.
When this week's game - Conarium - went up for free, they teased next week's as they often do. But there was something different, as it's just an image without a name.
Free between 19th and 26th of September, will be ? ? ? - or, if you look at the small image above, saying "Coming Soon", it appears to be a whole bunch of games featuring DC Comics' Dark Knight. See it, bigger, here:
The image is titled "WB_Batman_Teaser_Art", and features key art from the three LEGO Batmans, as well as three Batman Arkham games. What exactly this means for next week's freebie is, obviously, a mystery, but it seems unlikely that they would give users six games in one week.
In any case, it seems clear that LEGO Batman, LEGO Batman 2 DC Super Heroes, LEGO Batman 3: Beyond Gotham, Batman: Arkham Asylum, Batman: Arkham City, and Batman: Arkham Knight are coming to Epic Game Store next week, just missing Batman Day on 15th September.