User blog comment:Gourgeist/Shingeki no Kyojin Chapter 63 Review: Chains/@comment-177.224.167.88-20141118064736/@comment-99.120.99.33-20141205064509

It's the serum that Grisha injected in Eren that gave him the titan powers. I think it may be the reason why Reiner, Berthold and Annie were sent inside the wall to capture him. Grisha could be from the same town as Reiner and his friends. I don't know.