The limited edition of volume 12 of manga "Shingeki no Kyojin
", to be released on December 9th, will contain an OVA. The anime will be about an old journal found by Levi and Erwin when they conduct the surveillance operation outside the wall. The contents of "Ilse's Journal" result in some unexpected actions from Hange.
Source: Shingeki no Kyojin official website
Shingeki no Kyojin OVA